Structure manufacturing method including surface photoelectrochemical etching and structure manufacturing device

ABSTRACT

A process of preparing a wafer having a diameter of two inches or more, at least a surface of the wafer being formed from a group III nitride crystal, including preparing an alkaline or acidic etching liquid containing a peroxodisulfate ion as an oxidizing agent that accepts an electron, accommodating the wafer such that the surface of the wafer is immersed in the etching liquid such that the surface of the wafer is parallel with a surface of the etching liquid; and radiating light from the surface side of the etching liquid onto the surface of the wafer without agitating the etching liquid. First and second etching areas disposed at an interval from each other are defined on the surface of the wafer. In the process of radiating the light onto the surface of the wafer, the light is radiated perpendicularly onto surfaces of the first and second etching areas.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 37 U.S.C. § 371 toInternational Patent Application No. PCT/JP2019/050452, filed Dec. 24,2019, which claims priority to and the benefit of Japanese PatentApplication No. 2018-242924, filed on Dec. 26, 2018. The contents ofthese applications are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a structure production method andstructure production apparatus.

DESCRIPTION OF RELATED ART

Group III nitrides such as gallium nitride (GaN) are used as a materialfor producing semiconductor devices such as light-emitting elements andtransistors, and are also attracting attention as a material formicro-electro-mechanical systems (MEMS).

Etching that involves anodic oxidation (also referred to as“photo-electrochemical (PEC) etching” below) is being proposed as atechnique to etch group III nitrides such as GaN (see, for example,non-Patent Document 1). PEC etching is preferable because it is a typeof wet etching that causes less damage compared to ordinary dry etchingand also because the device used in the etching is more simple comparedto special dry etching techniques that are designed to cause lessdamage, such as neutral-beam etching (see, for example, non-PatentDocument 2) and atomic layer etching (see, for example, non-PatentDocument 3). Much is still unknown, however, about what ways group IIInitrides such as GaN can be processed using PEC etching.

There have been no studies conducted on problems related to when atechnique for producing a structure by carrying out PEC etching on agroup III nitride is implemented on a mass-production scale. Forexample, there have been no studies conducted on a technique forenhancing etching in-plane uniformity in PEC etching performed on awafer having a large diameter (e.g. a diameter of two inches or more).To cite another example, there have been no studies conducted on atechnique for performing PEC etching while limiting variations inetching conditions in terms of time.

-   Non-patent Document 1: J. Murata et al., “Photo-electrochemical    etching of free-standing GaN wafer surfaces grown by hydride vapor    phase epitaxy”, Electrochimica Acta 171 (2015) 89-95.-   Non-Patent Document 2: S. Samukawa, J J A P, 45(2006)2395.-   Non-Patent Document 3: T. Faraz, E C S J. Solid Stat.    Scie.&Technol., 4, N5023 (2015)).

One objective of the present invention is to provide a technique thatmakes it possible to enhance etching in-plane uniformity in PEC etchingperformed on a wafer, at least the surface of which is formed from agroup III nitride crystal and which has a large diameter of, forexample, two inches or more.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a structure productionmethod including

a process of preparing a wafer having a diameter of two inches or more,at least a surface of the wafer being formed from a group III nitridecrystal,

a process of preparing an alkaline or acidic etching liquid in acontainer, the etching liquid containing a peroxodisulfate ion as anoxidizing agent that accepts an electron,

a process of accommodating the wafer in the container in a condition inwhich the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with a surface ofthe etching liquid, and

a process of radiating light from the surface side of the etching liquidonto the surface of the wafer without agitating the etching liquid,

wherein

a first etching area and a second etching area are defined on thesurface of the wafer, the first and second etching areas being disposedat an interval from each other, the first and second etching areas beingareas where the group III nitride crystal is to be etched due to thesurface of the wafer being irradiated with the light in a condition inwhich the surface is immersed in the etching liquid, and

in the process of radiating the light onto the surface of the wafer, thelight is radiated perpendicularly onto each of a surface of the firstetching area and a surface of the second etching area.

Another aspect of the present invention provides a structure productionapparatus including

a container configured to accommodate a wafer and an alkaline or acidicetching liquid, the wafer having a diameter of two inches or more, atleast a surface of the wafer being formed from a group III nitridecrystal, the etching liquid containing a peroxodisulfate ion as anoxidizing agent that accepts an electron, and

a light irradiation device configured to radiate light from a surfaceside of the etching liquid onto the surface of the wafer, wherein

the container accommodates the wafer inside the container in a conditionin which the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with the surfaceof the etching liquid, and retains the etching liquid in a manner suchthat the light is radiated onto the surface of the wafer in a conditionin which the etching liquid is not agitated, and

the light irradiation device radiates the light perpendicularly onto asurface of each of a first etching area and a second etching area, thefirst and second etching areas being defined on the surface of thewafer, the first and second etching areas being disposed at an intervalfrom each other, the first and second etching areas being areas wherethe group III nitride crystal is to be etched due to the surface of thewafer being irradiated with the light in a condition in which thesurface is immersed in the etching liquid.

Provided is a technique that makes it possible to enhance etchingin-plane uniformity in PEC etching performed on a wafer at least thesurface of which is formed from a group III nitride crystal and whichhas a large diameter of, for example, two inches or more.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A through 1G are schematic sectional diagrams illustrating amethod for producing a GaN material (substrate) according to a firstembodiment of the present invention;

FIG. 2 is a schematic structural diagram illustrating an example of anHVPE device;

FIG. 3 is a schematic sectional diagram illustrating a GaN material(epitaxial substrate) according to a second embodiment;

FIG. 4 is a schematic structural diagram illustrating an example of anelectrochemical cell in a first experimental example;

FIG. 5 is a timing chart illustrating a sequence of PEC etching in thefirst experimental example;

FIG. 6 is a graph illustrating a relationship between an amount ofcharge per unit area consumed by anodic oxidation and etched depth inthe first experimental example;

FIG. 7 is a graph illustrating a relationship between etched depth andprofile roughness Ra of a bottom face of a formed recess in the firstexperimental example;

FIG. 8 is a graph illustrating a relationship between an etching voltageand profile roughness Ra in the first experimental example in a casewhere the etched depth is 2 μm;

FIG. 9 is a graph in which etching voltage 1 V and the vicinity thereofin FIG. 8 are enlarged;

FIGS. 10A through 10C are SEM images of bottom faces of recesses formedin the first experimental example in cases where etching voltages areset to 3 V, 2 V, and 1 V, and FIG. 10D is a SEM image of a surface inthe first experimental example in a case where no etching is carriedout;

FIGS. 11A through 11D are optical microscopic images of bottom faces ofrecesses formed in the first experimental example in cases where etchingvoltages are set to 3 V, 2 V, 1 V, and 0 V;

FIGS. 12A through 12D are AFM images of the bottom faces of the recessesformed in the first experimental example in the cases where the etchingvoltages are set to 3 V, 2 V, 1 V, and 0 V;

FIG. 13 illustrates PL emission spectra in the first experimentalexample in the cases where no etching is carried out and where theetching voltages are set to 0 V, 1 V, 2 V, and 3 V;

FIGS. 14A through 14C are schematic sectional diagrams illustrating partof a method for producing a semiconductor device employing a GaNmaterial (epitaxial substrate) according to a third embodiment;

FIGS. 15A and 15B are SEM images in which cylindrical protrusions formedin a second experimental example (fourth embodiment) are viewed in anoverhead view and a side view;

FIG. 16 is a SEM image in which a mask, a pn junction, and the vicinitythereof in the cylindrical protrusion formed in the second experimentalexample (fourth embodiment) are illustrated in enlarged fashion;

FIG. 17 is a SEM image in which cylindrical recesses formed in a thirdexperimental example (fifth embodiment) are viewed in an overhead view;

FIGS. 18A through 18C are SEM images illustrating cross-sections oftrenches (target depth: 7.7 μm) formed in a fourth experimental example(sixth embodiment), where the cross-sections are taken along a directionperpendicular to the length direction of the trenches;

FIGS. 19A through 19C are SEM images illustrating cross-sections oftrenches (target depth: 33 μm) formed in the fourth experimental example(sixth embodiment), where the cross-sections are taken along a directionperpendicular to the length direction of the trenches;

FIG. 20 is a graph illustrating a relationship between etched depth andan aspect ratio of a trench formed in the fourth experimental example(sixth embodiment);

FIG. 21 is a schematic diagram illustrating an example of a situationwhere PEC etching is carried out to penetrate through a GaN material;

FIG. 22A is a schematic diagram illustrating an example of a situationwhere a filling member is loaded into a recess formed using PEC etching,and FIG. 22B is a schematic diagram illustrating an example of asituation where a filling member is loaded on the outside of aprotrusion formed using PEC etching;

FIG. 23 is an optical microscopic photograph illustrating the result ofan experiment in which a rectangular GaN material, the side faces ofwhich are constituted by an a face and an m face perpendicular to the aface, is etched with hot phosphoric acid sulfuric acid;

FIGS. 24A through 24D are schematic diagrams each illustrating anexample of a manner in which PEC etching according to a seventhembodiment is carried out;

FIGS. 25A and 25B are timing charts each illustrating an example of amanner in which the PEC etching according to the seventh embodiment iscarried out;

FIG. 26 is a schematic diagram illustrating an example of a manner inwhich PEC etching according to a first variation of the seventhembodiment is carried out;

FIG. 27 is a schematic diagram illustrating an example of a manner inwhich PEC etching according to a second variation of the seventhembodiment is carried out;

FIG. 28 is schematic diagram illustrating an example of a manner inwhich PEC etching according to an eighth embodiment is carried out;

FIG. 29 is a timing chart illustrating an example of a manner in whichthe PEC etching according to the eighth embodiment is carried out;

FIG. 30 is a timing chart illustrating an example of a manner in whichPEC etching according to a first variation of the eighth embodiment iscarried out;

FIG. 31 is a timing chart illustrating an example of a manner in whichPEC etching according to a second variation of the eighth embodiment iscarried out;

FIGS. 32A and 32B are schematic diagrams respectively illustratingexamples of manners in which PEC etching according to third and fourthvariations of the eighth embodiment is carried out;

FIG. 33 is a schematic diagram illustrating an example of a manner inwhich PEC etching according to a fifth variation of the eighthembodiment is carried out;

FIGS. 34A and 34B are schematic diagrams each illustrating an example ofa manner in which PEC etching according to a ninth embodiment is carriedout; and

FIG. 35 is a timing chart illustrating an example of a manner in whichthe PEC etching according to the ninth embodiment is carried out.

DETAILED DESCRIPTION OF THE INVENTION

A gallium nitride (GaN) material 100 according to an embodiment of thepresent invention will be described. Also will be described etching thatinvolves anodic oxidation (also referred to as “photo-electrochemical(PEC) etching” below) and that is to be performed on the GaN material100. PEC etching can be used as a method for processing the GaN material100 and also as a method for evaluating a characteristic of the GaNmaterial 100.

First Embodiment

A first embodiment will be described first. The first embodimentillustrates a GaN substrate 10 (also referred to as “substrate 10”below) as an example of the GaN material 100. FIGS. 1A through 1G areschematic sectional diagrams illustrating the process of producing thesubstrate 10 using a void-assisted separation (VAS) method. First, anunderlying substrate 1 is prepared, as illustrated in FIG. 1A. Asapphire substrate is illustrated as an example of the underlyingsubstrate 1.

Next, an underlying layer 2 is formed on the underlying substrate 1, asillustrated in FIG. 1B. The underlying layer 2 may be constituted by,for example, a stack including a buffer layer constituted by lowtemperature-grown GaN and a single crystal layer of GaN. The bufferlayer and the single crystal layer may be formed by, for example,metalorganic vapor phase epitaxy (MOVPE). Trimethyl gallium (TMG) may beused as an example of gallium (Ga) raw material and ammonia (NH₃) may beused as an example of nitrogen (N) raw material. The thicknesses of thebuffer layer and the single crystal layer may respectively be, forexample, 20 nm and 0.5 μm.

Next, a metal layer 3 is formed on the underlying layer 2, asillustrated in FIG. 1C. The metal layer 3 may be formed by, for example,vapor deposition of titanium (Ti) in an amount of a thickness of 20 nm.

Next, heat treatment is carried out to nitride the metal layer 3 so asto form a nanomask 3 a and to form voids in the underlying layer 2 so asto form a void-including layer 2 a, as illustrated in FIG. 1D. The heattreatment may be carried out in the following way, for example. Theunderlying substrate 1 on which the underlying layer 2 and the metallayer 3 have been formed is put in an electric furnace and placed on asusceptor having a heater. The underlying substrate 1 is then heated inan atmosphere containing hydrogen gas (H₂ gas) or hydride gas.Specifically, heat treatment may be carried out, for example, for 20minutes in an H₂ gas flow containing 20% of NH₃ gas as nitrided gas andat a prescribed temperature, for example, a temperature between 850° C.and 1,100° C. (inclusive).

Nitridation of the metal layer 3 due to such heat treatment results inthe formation of a nanomask 3 a, the surface of which has highly denselyformed fine pores. Part of the underlying layer 2 is etched through thefine pores of the nanomask 3 a, resulting in the formation of voids inthe underlying layer 2 and thus in the formation of the void-includinglayer 2 a. In this way, a substrate 4 in which voids are formed(“void-formed substrate 4” below) is produced that includes thevoid-including layer 2 a and the nanomask 3 a formed on the underlyingsubstrate 1.

Preferably, the heat treatment is carried out in the following way. Theheat treatment is carried out such that the “void formation rate (volumeporosity)” indicative of the proportion in volume of the voids in thevoid-including layer 2 a is uniform on the underlying substrate 1 in thecircumferential direction. Specifically, the susceptor on which theunderlying substrate 1 is placed may be rotated, for example, so as tocarry out heat treatment uniformly in the circumferential direction. Itis also possible to, for example, adjust the degree to which the heaterheats the face of the underlying substrate 1, thereby making thetemperature distribution in the epitaxial substrate uniform in thecircumferential direction. Furthermore, the heat treatment is carriedout such that the void formation rate in the void-including layer 2 aincreases steadily from the center of the underlying substrate 1 towardthe outer circumference thereof in the radial direction. Specifically,the degree to which the heater heats the face of the underlyingsubstrate 1 may be adjusted, for example, so that the temperature of theunderlying substrate 1 increases monotonically from the center of theunderlying substrate 1 toward the outer circumference thereof in theradial direction.

Next, a crystal 6 is grown on the nanomask 3 a of the void-formedsubstrate 4, as illustrated in FIG. 1E. The crystal 6 is grown by a gasphase method, specifically by a hydride vapor phase epitaxy (HVPE)method. In this regard, an HVPE device 200 will be now described. FIG. 2is a schematic structural diagram illustrating an example of the HVPEdevice 200.

The HVPE device 200 is formed from a heat-resistant material, such asquartz, and includes a hermetic container 203, the interior of which isprovided with a film formation chamber 201. A susceptor 208 serving tohold the void-formed substrate 4 subject to treatment is provided insidethe film formation chamber 201. The susceptor 208 is connected to arotary shaft 215 of a rotary mechanism 216 and is configured to berotatable. Gas supply tubes 232 a through 232 c serving to supplyhydrochloric acid (HCI) gas, NH₃ gas, and nitrogen gas (N₂ gas) into thefilm formation chamber 201 are connected to one end of the hermeticcontainer 203. A gas supply tube 232 d serving to supply hydrogen (H₂)gas is connected to the gas supply tube 232 c. Flow rate control devices241 a through 241 d and valves 243 a through 243 d are providedrespectively on the gas supply tubes 232 a through 232 d in that orderfrom an upstream side. A gas generation device 233 a that accommodates aGa melt as raw material is provided downstream of the gas supply tube232 a. A nozzle 249 a is connected to the gas generation device 233 a.The nozzle 249 a serves to supply gallium chloride (GaCl) gas producedby reaction between HCl gas and the Ga melt toward the void-formedsubstrate 4 held on the susceptor 208. Nozzles 249 b and 249 c areconnected respectively to the downstream side of the gas supply tubes232 b and 232 c. The nozzles 249 b and 249 c serve to supply the variousgases supplied from the gas supply tubes 232 b and 232 c toward thevoid-formed substrate 4 held on the susceptor 208. A gas discharge tube230 serving to discharge the gas inside the film formation chamber 201is provided on the other end of the hermetic container 203. A pump 231is provided on the gas discharge tube 230. Zone heaters 207 serving toheat the interior of the gas generation device 233 a and the void-formedsubstrate 4 held on the susceptor 208 to desired temperatures areprovided around the outer periphery of the hermetic container 203, and atemperature sensor 209 serving to measure the temperature inside thefilm formation chamber 201 is provided in the hermetic container 203.The members included in the HVPE device 200 are connected to acontroller 280 constituted by a computer and are configured such thatthe processing procedures and processing conditions described later arecontrolled by a program that is executed on the controller 280.

The crystal 6 epitaxial processing may, for example, be implemented bythe processing procedures below using the HVPE device 200. First, Ga isaccommodated in the gas generation device 233 a as raw material. Thevoid-formed substrate 4 is placed and held on the susceptor 208. Then, agas mixture containing H₂ gas and N₂ gas is supplied into the filmformation chamber 201 while the film formation chamber 201 is beingheated and gas is being discharged therefrom. In a state where the filmformation temperature and the film formation pressure inside the filmformation chamber 201 have reached the desired temperature and pressureand the atmosphere inside the film formation chamber 201 has become thedesired atmosphere, gas supply from the gas supply tubes 232 a and 232 bis carried out such that GaCl gas and NH₃ gas are supplied to thevoid-formed substrate 4 as film formation gases.

The processing conditions for the crystal 6 epitaxial processing may beas follows, for example.

Growth temperature Tg: 980° C.-1,100° C., preferably 1,050° C.-1,100° C.

Pressure inside film formation chamber 201: 90-105 kPa, preferably 90-95kPa

GaCl gas partial pressure: 1.5-15 kPa

NH₃ gas partial pressure/GaCl gas partial pressure: 4-20

N₂ gas partial pressure/H₂ gas partial pressure: 1-20

In the epitaxial processing, the GaN crystal that started to grow fromthe void-including layer 2 a appear on the surface through the finepores of the nanomask 3 a, resulting in the formation of initial nucleion the nanomask 3 a. The growth of the initial nuclei in the thicknessdirection (vertical direction) as well as the in-plane direction(horizontal direction) and bonding of the same in the plane results inthe formation of the crystal 6 constituted by a continuous film formedfrom a GaN single crystal. In areas where no initial nuclei are formed,voids 5 are formed between the nanomask 3 a and the crystal 6 accordingto the presence of the voids in the void-including layer 2 a. Since thevoid formation rate in the void-including layer 2 a is controlled in theaforementioned way, the voids 5 are formed uniformly in thecircumferential direction and become larger from the center toward theoutside in the radial direction.

In this epitaxial processing, the crystal 6 is grown on the void-formedsubstrate 4, so the distribution of initial nucleus generation densitycan be made more uniform compared to the epitaxially lateral overgrowth(ELO) method employing a stripe mask or other such methods in which thedistribution of initial nucleus generation density is made non-uniformto create dislocation concentration areas in which local dislocationdensity is extremely high (for example, 1×10⁷/cm² or more). Accordingly,in-plane maximum dislocation density can be limited to a low value (forexample, lower than 1×10⁷/cm²).

Moreover, in the epitaxial processing, GaN crystal growing from thevoid-including layer 2 a appears on the surface through the fine poresof the nanomask 3 a more readily toward the center in the radialdirection where the void formation rate is lower, thus initial nucleitend to form earlier toward the center. In other words, GaN crystalgrowing from the void-including layer 2 a appears on the surface throughthe fine pores of the nanomask 3 a less readily toward the outside inthe radial direction where the void formation rate is higher, thusinitial nuclei tend to form later toward the outside. Accordingly, thegrowth and boding of the initial nuclei can be made to progressgradually from the center toward the outside in the radial direction, soit is easier to grow the initial nuclei into a larger size. Furthermore,since such growth and bonding of the initial nuclei can be made toprogress uniformly in the circumferential direction. As a result,crystal quality such as the in-plane uniformity of the crystal 6 can beenhanced.

Preferably, the crystal 6 grown has a thickness that enables at leastone independent substrate 10 to be obtained from the crystal 6, forexample, a thickness of 0.2 mm or more. There are no particularlimitations on the upper limit of the thickness of the crystal 6 grown.

Next, the crystal 6 is peeled off from the void-formed substrate 4, asillustrated in FIG. 1F. This peeling is achieved during growth of thecrystal 6 or in the process of cooling the interior of the filmformation chamber 201 after completion of the growth of the crystal 6 asa result of the crystal 6 peeling off spontaneously from the void-formedsubstrate 4; here, the voids 5 formed between the crystal 6 and thenanomask 3 a serve as the boundary of peeling.

Force, which is the result of mutual attraction of the initial nucleibonding together during growth of the crystal 6, occurs in the crystal6, so the crystal 6 contains tensile stress thereinside. Due to thistensile stress, the crystal 6 having peeled off warps in the manner ofthe growth-side surface thereof being depressed. Accordingly, the c faceof the GaN single crystal constituting the crystal 6 that has peeled offcurves in the form of a depressed spherical surface relative to animaginary plane that is perpendicular to a direction normal to thecenter of a principal face 6 s of the crystal 6. “Spherical surface” asreferred to herein means a curved surface that approximates a sphericalsurface. “Approximates a spherical surface” as referred to herein meansapproximating the spherical surface of a true circle or an ellipse withan error falling within a prescribed error range.

Since the voids 5 are formed uniformly in the circumferential directionand so as to become larger from the center toward the outside in theradial direction, the crystal 6 can peel off uniformly from the outercircumference toward the center of the void-formed substrate 4 in thecircumferential direction. Accordingly, natural peeling that is inconformity with the warping shape of the crystal 6 can be achieved, andtherefore, the generation of unwanted stress that result from peelingcan be limited. So, in this production method, as described above,crystal growth is carried out employing a VAS method while controllingthe void formation rate in the aforementioned way; thus, a crystal 6with enhanced crystal quality such as in-plane uniformity can beobtained.

After completion of the growth of the crystal 6 having a prescribedthickness, supply of the various gases used for the epitaxial processingis stopped and the atmosphere inside the film formation chamber 201 issubstituted with N₂ gas to recover the atmospheric pressure. Thevoid-formed substrate 4 and the crystal 6 are drawn out of the filmformation chamber 201 after the temperature inside the film formationchamber 201 has been lowered to a temperature at which such draw-outwork is possible.

Next, the crystal 6 is machined (for example, cut with a wire saw) andrubbed, as appropriate, to obtain one or more substrates 10 from thecrystal 6, as illustrated in FIG. 1G. The crystal face with a low indexclosest to the principal face 10 s of the substrate 10 illustrated inFIG. 1G as an example is the c face.

The substrate 10 is produced in the aforementioned way. In addition tohaving a maximum dislocation density limited to be lower than 1×10⁷/cm²(i.e. having no areas where the dislocation density reaches or exceeds1×10⁷/cm²), the substrate 10 has high in-plane uniformity. The followingdescribes an example of a specific condition that represents a “limited”dislocation density of the substrate 10. In the principal face 10 s ofthe substrate 10, measurement is carried out using thecathodoluminescence (CL) method by scanning a 500 μm-diameterobservation area within a 3 mm-square measurement area. The measurementis carried out on more or less ten such observation areas. At this time,the maximum dislocation density is lower than 1×10⁷/cm², and in apreferred example, 5×10⁶/cm² or lower. Preferably, an averagedislocation density is 3×10⁶/cm² or lower, for example. There are noparticular limitations on a minimum dislocation density. The ratio of amaximum dislocation density to a minimum dislocation density mayincrease in conformity with a decrease in a minimum dislocation density,and as a rough standard, may be 100:1 or less, or 10:1 or less, forexample.

The inventors of the present invention arrived at the finding that thesubstrate 10 constituting the GaN material 100 according to the firstembodiment is a suitable material to be processed using PEC etching toform a recess with superior internal flatness (i.e. flatness of theinternal faces of the recess). The PEC etching and the internal flatnessof the formed recess will be described in detail later. The principalface 10 s may be used as an example of a face in which such a recess isto be formed using PEC etching (also referred to as an “etching face”below).

Impurities may be added to the substrate 10. If impurities are to beadded, a gas supply tube and the like for supplying gas that containssuch impurities may be additionally provided in the HVPE device 200illustrated in FIG. 2. Examples of such impurities include those servingto impart electroconductivity to the substrate 10, and may be n-typeimpurities, for example. Usable examples of n-type impurities includesilicon (Si) and germanium (Ge). If Si is to be added, for example, thendichlorosilane (SiH₂Cl₂) may be used as Si raw material, for example.Impurities may also be those serving to impart semi-insulatingproperties to the substrate 10, for example.

Second Embodiment

Next, a second embodiment will be described. In the second embodiment, afirst experimental example will also be described together. In thesecond embodiment, as illustrated in FIG. 3, a stack 30 (also referredto as an “epitaxial substrate 30” below), which includes a substrate 10and a GaN layer 20 that is epitaxially grown (also referred to as an“epitaxial layer” below) on the substrate 10, is illustrated as anexample of the GaN material 100. For the substrate 10, the substrate 10described in the first embodiment may be used preferably.

The second embodiment features an example case where n-type impuritiesare added to both the substrate 10 and the epitaxial layer 20. Althoughthere are no particular limitations on the constitution of the substrate10 and the epitaxial layer 20, the following illustrates a possibleexample. In the substrate 10, Si may be added as an example of n-typeimpurities at a concentration of between 1×10¹⁸/cm³ and 1×10¹⁹/cm³(inclusive). In the epitaxial layer 20, Si, for example, may be added ata concentration of between 3×10¹⁵/cm³ and 5×10¹⁶/cm³ (inclusive). Whenthe epitaxial substrate 30 is to be used as a material for asemiconductor device, the substrate 10 would presumably be used as acontact layer for contact with an electrode and the epitaxial layer 20would presumably be used as a drift layer, and it is preferred that theconcentration of the n-type impurities added to the epitaxial layer 20be lower than that of the substrate 10 from the view point of anincrease in pressure-withstanding performance. Although not particularlylimited, the thickness of the substrate 10 may be 400 for example. Thethickness of the epitaxial layer 20 may be between 10 μm and 30 μm(inclusive), for example. The epitaxial layer 20 may be constituted by astack of multiple GaN layers having differing n-type impurityconcentrations.

The epitaxial layer 20 may be grown on the principal face 10 s of thesubstrate 10 by MOVPE, for example. TMG may be used as an example of Garaw material, NH₃ may be used as an example of N raw material, andmonosilane (SiH₄) may be used as an example of Si raw material. Theepitaxial layer 20 grows incorporating the crystallinity of thesubstrate 10, so similarly to the substrate 10, has its maximumdislocation density limited to be lower than 1×10⁷/cm² while also havinghigh in-plane uniformity.

The inventors of the present invention arrived at the finding that theepitaxial substrate 30 constituting the GaN material 100 according tothe second embodiment is a suitable material to be processed using PECetching to form a recess with superior internal flatness, as will bedescribed in detail in the first experimental example below.

Now, PEC etching and the internal flatness of the recess formed usingPEC etching will be described along the first experimental example. FIG.4 is a schematic structural diagram illustrating an example of anelectrochemical cell 300 used in PEC etching. A container 310 stores anelectrolyte solution 320. As an example of the electrolyte solution 320,a 0.01 M sodium hydroxide (NaOH) solution to which 1% of Triton(registered trademark) X-100 (by Sigma Chemical) has been added as asurfactant may be used.

A platinum (Pt) coil may be used as an example of a cathode electrode330. The cathode electrode 330 is disposed in the electrolyte solution320. The GaN material 100 is used as an anode electrode 340. Thecontainer 310 has an opening 311, and a sealing ring 312 is disposed soas to surround the opening 311 and be interposed between the container310 and the GaN material 100. The GaN material 100 is disposed so as toclose an opening 313 of the sealing ring 312 located on the oppositeside from the container 310. Accordingly, the GaN material 100 contactsthe electrolyte solution 320 filling the hole of the sealing ring 312.An ohmic contact probe 341 is attached to the GaN material 100 (anodeelectrode 340) so as not to contact the electrolyte solution 320.

The cathode electrode 330 and the ohmic contact probe 341 attached tothe anode electrode 340 are connected to each other by a wire 350. Avoltage source 360 is inserted partway through the wire 350. The voltagesource 360 applies a prescribed etching voltage between the cathodeelectrode 330 and the anode electrode 340 at a prescribed timing.

A light source 370 is disposed on the outside of the container 310. Thelight source 370 emits ultraviolet (UV) light 371 having a prescribedirradiation intensity at a prescribed timing. Usable examples of thelight source 370 include mercury xenon (Hg—Xe) lamps (for example,LIGHTNINGCURE (registered trademark) L9566-03 (by Hamamatsu PhotonicsK.K.)). A window 314 allowing the UV light 371 to pass through isprovided on the container 310. The UV light 371 emitted from the lightsource 370 passes through the window 314, the electrolyte solution 320,the opening 311 of the container 310, and the opening 313 of the sealingring 312 and irradiates the GaN material 100 (anode electrode 340). Apump 380 is attached to the container 310. The pump 380 agitates theelectrolyte solution 320 in the container 310 at a prescribed timing.

As the anode electrode 340 is irradiated with the UV light 371, thefollowing reaction progresses in the anode electrode 340 and the cathodeelectrode 330. In the anode electrode 340, holes resulting from the UVlight 371 irradiation resolve the GaN into Ga³+ and N₂ (chem. 1), andmoreover, Ga³+ is oxidized by the OH⁻ group (chem. 2), resulting in thegeneration of gallium oxide (Ga₂O₃). As a result of the generated Ga₂O₃being dissolved by the NaOH solution (electrolyte solution 320), theanode electrode 340, i.e. the GaN material 100, is etched. PEC etchingis carried out in this way.

(Anode Reaction)

$\begin{matrix} {{{GaN}(s)} + {3h^{+}}}arrow{{Ga}^{3 +} + {\frac{1}{2} {N_{2}(g)}\uparrow }}  & \lbrack {{Chemical}\mspace{14mu}{Formula}\mspace{14mu} 1} \rbrack \\ {{Ga}^{3 +} + {3{OH}^{-}}}arrow{{\frac{1}{2}{Ga}_{2}{O_{3}(s)}} + {\frac{3}{2}H_{2}{O(l)}}}  & \lbrack {{Chemical}\mspace{14mu}{Formula}\mspace{14mu} 2} \rbrack \\ {{H_{2}{O(l)}} + {2h^{+}}}arrow{\frac{1}{2} {O_{2}(g)}\uparrow{+ 2} H^{+}}  & \lbrack {{Chemical}\mspace{14mu}{Formula}\mspace{14mu} 3} \rbrack\end{matrix}$(Cathode Reaction)2H₂O(I)+2e ⁻→2OH⁻+H₂(g)↑  [Chemical Formula 4]

In the first experimental example, specifically, the epitaxial substrate30 was used as the GaN material 100 constituting the anode electrode340. For more detailed description, the epitaxial layer 20 wasirradiated with the UV light 371 while the epitaxial layer 20 side ofthe epitaxial substrate 30 was contacting the electrolyte solution 320,thereby causing anodic oxidation at the epitaxial layer 20 to etch thesame. In other words, the principal face 20 s of the epitaxial layer 20was used as an etching face.

In the first experimental example, a GaN substrate having a Siconcentration of 1 to 2×10¹⁸/cm³ was used as the substrate 10. Theepitaxial layer 20 was formed by growing a GaN layer having a Siconcentration of 2×10¹⁸/cm³ and a thickness of 2 μm and a GaN layerhaving a Si concentration of 9×10¹⁵/cm³ and a thickness of 13 μm on thesubstrate 10 by MOVPE. The overall size of the epitaxial substrate 30was set to be a diameter of two inches (5.08 cm) and the size of thearea to be etched by the electrolyte solution 320 coming into contactwith the epitaxial layer 20, i.e. the size of the opening 313 of thehole of the sealing ring 312, was set to a diameter of 3.5 mm.

The irradiation intensity at the etching face was set to be 9 mW/cm². UVlight irradiation and application of the etching voltage were repeatedintermittently by repeating a set consisting of: carrying out UV lightirradiation and application of the etching voltage simultaneously for 13seconds; and then stopping the same for 9 seconds. In other words,pulsed anodic oxidation was carried out. The etching voltage was changedfrom 0 V to 1 V, 2 V, and 3 V to confirm changes resulting therefrom inthe flatness of the bottom face of the recess formed using PEC etching.Results of the first experimental example will be described below withreference to FIGS. 5 through 13.

In commercially available devices designed to carry out PEC etching onvarious materials, it is common to set the etching voltage to a highvoltage exceeding 3 V. A characteristic of this experimental examplelies in that a low etching voltage range of 3 V or lower is adopted.

FIG. 5 is a timing chart illustrating a sequence of PEC etching. Asmentioned above, UV light irradiation and application of the etchingvoltage were repeated intermittently by repeating a set consisting of:carrying out UV light irradiation (“Lump” in the drawings) andapplication of the etching voltage (“Vetch” in the drawings)simultaneously for 13 seconds; and then stopping the same for 9 seconds.The pump 380 is used to agitate the electrolyte solution 320 (“Pump” inthe drawings) within the period in which UV light irradiation and theapplication of the etching voltage are stopped, more specifically, inthe first 5 seconds of this period.

The lower part of FIG. 5 illustrates the etching currents correspondingto cases where the etching voltages of 0 V, 1 V, 2 V, and 3 V were used.For all etching voltages, an etching current flows during the UV lightirradiation period and does not flow during the UV light stoppageperiod. During the UV light irradiation period, an etching current flowsas a result of the OH-group reaching the anode electrode 340 accordingto the aforementioned anode reaction, even if the etching voltage is 0V. An increase in the etching voltage results in an increase in a driveforce of attracting the OH⁻ group toward the anode electrode 340,leading to an increase in the etching current.

FIG. 6 is a graph illustrating a relationship between an amount ofcharge per unit area consumed by anodic oxidation and etched depth (etchdepth, etching depth). The result corresponding to the 0 V-etchingvoltage is illustrated with a square plot, and likewise, 1 V-etchingvoltage: triangular plot, 2 V-etching voltage: rhombic plot, and 3V-etching voltage: circular plot. The same plotting is used in FIG. 7that will be described later.

Etched depth was measured using a process profiler (Sloan, Dektak³ ST).It can be seen that the etched depth changes linearly in relation to theconsumed amount of charge. The etched depth W_(r) is expressed as[Formula 1]W _(r) =M/zFρ∫Jdt  (1)according to the Faraday's law. Here, “M” expresses the molecular weightof GaN, “z” expresses the required valence for anodic oxidation per 1mol of GaN, “F” expresses a Faraday constant, “ρ” expresses the densityof GaN, and “J” expresses etching current density. According toexpression (1), the hole necessary for anodic oxidation of 1 mol of GaNis z=5.3-6.8 mol. For the generation of Ga₂O₃ (chem. 1 and chem. 2)alone, z=3 mol. Thus, this result indicates that in the anode electrode340, oxygen gas is generated in addition to the generation of Ga₂O₃ andthe hole is consumed.

FIG. 7 is a graph illustrating a relationship between etched depth andcalculated average profile roughness Ra (may be referred to simply as“profile roughness Ra” in this specification) of a bottom face of aformed recess in the first experimental example. The profile roughnessRa was measured with a contact-type process profiler (Sloan, Dektak³ST). In the measurement with the contact-type process profiler, theprofile roughness Ra was calculated by using, within the evaluationlength of 500 μm, 100 μm as a reference length. In other words, themeasurement length for obtaining the profile roughness Ra was set to be100 μm. In FIG. 7, the results obtained with the etching voltage of 1 Vare illustrated in enlarged fashion as representative results, andtogether therewith, the results for the etching voltages of 0 V, 2 V,and 3 V are illustrated in reduced fashion in the lower right part.

It can be seen that in the range in which the etched depths are between0 μm and 10 μm (inclusive), the profile roughness Ra for the etchingvoltage of 1 V is remarkably small for all depths. For example, for theetched depth of 10 μm, while the profile roughness Ra for the etchingvoltage of 2 V is about 150 nm, the profile roughness Ra for the etchingvoltage of 1 V is as extremely small as 10 nm or less, specificallyabout 8 nm. In other words, it can be seen that the flatness of thebottom face of the recess formed with the etching voltage of 1 V isremarkably superior. It should be noted that an increase in the depth ofa recess tends to result in a decrease in the flatness of the bottomface.

FIG. 8 is a graph illustrating a relationship between an etching voltageand profile roughness Ra in a case where the etched depth is 2 μm (agraph in which the results for the etched depth of 2 μm in FIG. 7 areplotted anew). FIG. 9 is a graph in which the etching voltage of 1 V andthe vicinity thereof in FIG. 8 are enlarged.

The profile roughness Ra is 17 nm, 3.5 nm, 40 nm, and 70 nm for theetching voltages of 0 V, 1 V, 2 V, and 3 V, respectively. For theetching voltage of 1 V, a very flat bottom face with a profile roughnessRa of no more than 5 nm is obtained. In view of this result, an etchingvoltage for which the profile roughness Ra will be no more than 15 nm,for example, can be estimated to be a voltage falling within the rangeof 0.16 V to 1.30 V (inclusive), whereas an etching voltage for whichthe profile roughness Ra will be no more than 10 nm, for example, can beestimated to be a voltage falling within the range of 0.52 V to 1.15 V(inclusive).

FIGS. 10A through 10C are scanning electron microscopic (SEM) images ofbottom faces of recesses formed in cases where the etching voltages areset to 3 V, 2 V, and 1 V. FIG. 10D is a SEM image obtained in a casewhere no etching is carried out. It can be seen that the bottom face ofthe recess has superior flatness for the etching voltage of 1 V.

FIGS. 11A through 11D are optical microscopic images of bottom faces ofrecesses formed in cases where the etching voltages are set to 3 V, 2 V,1 V, and 0 V. The circular area illustrated in the left part of eachimage illustrates the etched area, i.e. the recess. It can be seen thatfor the etching voltage of 1 V, the bottom face of the recess hassuperior flatness over a wide area of, for example, 500 μm square ormore or, for example, 1 mm square or more.

FIGS. 12A through 12D are atomic force microscopic (AFM) images of thebottom faces of the recesses formed in the cases where the etchingvoltages are set to 3 V, 2 V, 1 V, and 0 V. It can be seen that thebottom face of the recess has superior flatness for the etching voltageof 1 V. For the etching voltage of 1 V, the calculated average surfaceroughness Ra for a 5 μm-square measurement area (area subject toevaluation) in the bottom face of the recess, as measured using AFM, is2.6 nm. Meanwhile, for the etching voltage of 0 V, nonuniformity isobserved in terms of the presence of both areas with relatively superiorflatness and areas with relatively inferior flatness. It is inferredthat the reason for this nonuniformity is that since no etching voltageis applied, the ease with which the OH⁻ group is supplied, i.e. the easewith which Ga₂O₃ is generated, differs from one area to another,resulting in the presence of areas that are etched more readily andareas that are etched less readily.

FIG. 13 is a graph illustrating how the photoluminescence (PL)characteristics of a GaN material change according to PEC etching andillustrates PL emission spectra in the cases where no etching is carriedout and where the etching voltages are set to 0 V, 1 V, 2 V, and 3 V.The peak intensity of a PL emission spectrum at a band edge of GaN(about 3.4 eV) will be referred to as “band-edge peak intensity” here.The band-edge peak intensities for all etching voltages have anintensity that is 90% or more in relation to the band-edge peakintensity for when no etching was carried out. In other words, when anyof those etching voltages are used, the rate of change (reduction) inband-edge peak intensity due to anodic oxidation is less than 10%. Thus,as can be seen, the PEC etching has demonstrated itself as being amethod with which GaN material can be processed with almost no damage tothe GaN crystal.

Results obtained in the first experimental example can be summarized asfollows. When a recess is formed in the GaN material 100 using PECetching while changing the etching voltage from 0 V to 1 V, 2 V, and 3V, the flatness of the bottom face of the recess is superior for theetching voltage of 1 V above all. It is inferred that if the etchingvoltage is excessively high, for example 2 V or 3 V, the etching isintense, and this leads to a decrease in the flatness of the bottom faceof the recess. Meanwhile, it is inferred that if the etching voltage isexcessively low, 0 V, then areas that are etched more readily and areasthat are etched less readily occur, and this also leads to a decrease inthe flatness of the bottom face of the recess.

It is inferred that if the etching voltage is about 1 V, the etching isappropriate, and this leads to an increase in the flatness of the bottomface of the recess. To give a specific example, for the purpose ofproviding a rough standard for obtaining a profile roughness Ra of about15 nm or less for the bottom face of a recess to be formed, it ispreferred that the etching voltage be a voltage falling within the rangeof 0.16 V to 1.30 V (inclusive). To give another example, for thepurpose of providing a rough standard for obtaining a profile roughnessRa of about 10 nm or less for the bottom face of a recess to be formed,it is preferred that the etching voltage be a voltage falling within therange of 0.52 V to 1.15 V (inclusive).

As described above, in the first experimental example, PEC etching thatresults in superior flatness could be carried out with an etchingvoltage of about 1 V. An etching voltage of about 1 V is significantlylower than etching voltages that are usually used for PEC etching, e.g.an etching voltage exceeding 3 V. It is considered that in order to makePEC etching with such a low etching voltage possible, it is preferredthat, primarily, the dislocation density of the GaN material 100 in theetching face be adequately low (for example, the dislocation density be,at most, smaller than 1×10⁷/cm², i.e. there be no areas having adislocation density of 1×10⁷/cm² or more); this is because in areaswhere the dislocation density is excessively high (for example,1×10⁷/cm² or more), the holes generated due to UV light irradiation aretrapped, which inhibits anodic oxidation. In addition, in order toachieve etching resulting in superior flatness with such a low etchingvoltage, it is preferred that the in-plane uniformity of the GaNmaterial 100 in the etching face be high so that nonuniformity in termsof the ease with which anodic oxidation occurs is limited.

In view of the above discussion, the profile roughness Ra of a bottomface of a recess formed using PEC etching can be used as an index forevaluating the characteristics (lowness of dislocation density andin-plane uniformity) of the GaN material 100. The GaN material 100according to the first and second embodiments, i.e. the substrate 10 andthe epitaxial substrate 30, is characterized by being a GaN material inwhich a recess with superior internal flatness can be formed using PECetching. Specifically, the GaN material 100 according to the first andsecond embodiments constitutes a GaN material having a low dislocationdensity and high in-plane uniformity to a degree such that, whenassuming a case where a recess with the depth of 2 μm is to be formedusing PEC etching (etching voltage=1 V) while carrying out UV lightirradiation, then the bottom face of the recess will be formed into aflat face having a profile roughness Ra of preferably 15 nm or less,more preferably 10 nm or less, yet more preferably 5 nm or less.

The surface of the GaN material 100 having undergone no etching(“non-etched surface” below) is flat to a degree such that the profileroughness Ra thereof is, for example, 0.5 nm. In other words, in amember obtained by forming a recess in the GaN material 100 using PECetching (also referred to as a “GaN member” below), the non-etchedsurface consisting of an upper face on the outside of the recess is flatto a degree such that the profile roughness Ra thereof is, for example,0.5 nm. The profile roughness Ra being preferably 15 nm or less, morepreferably 10 nm or less, yet more preferably 5 nm or less as describedabove implies that the profile roughness Ra of the bottom face of therecess of the GaN member (GaN material 100) is preferably 30 times orless, more preferably 20 times or less, yet more preferably 10 times orless than the profile roughness Ra of the surface (non-etched surface)on the outside of the recess. Note that the bottom face of a recess, theetched depth of which is shallower than 2 μm, can be said to be moreflat than the bottom face of a recess having a 2-μm etched depth. Thus,the aforementioned condition is applicable to the formation of not onlya recess having a 2 μm-etched depth but also to a recess having anetched depth of less than or equal to 2 μm.

For a bottom face of a recess formed using PEC etching as describedabove, damage to the GaN crystal caused by the etching is little. Thus,for the GaN member (GaN material 100), the band-edge peak intensity ofthe PL emission spectrum for the bottom face of a recess has anintensity that is 90% or more in relation to the band-edge peakintensity of the PL emission spectrum for the surface on the outside ofthe recess (non-etched surface).

For the evaluation method described hereabove, the formation of astructure is assumed in which the “recess” is one that has a bottomface, i.e. the GaN material 100 is not penetrated through, but whenactually carrying out processing using PEC etching, a structure may beformed where the GaN material 100 is penetrated through to form the“recess”.

The etching voltage conditions revealed in the first experimentalexample are considered to be valid as a rough standard for improving theinternal flatness of a recess for not only the PEC etching designed forthe GaN material 100 according to the first and second embodiments, butalso PEC etching that is performed on an area of a GaN material havingan adequately low dislocation density (for example, less than1×10⁷/cm²). That is to say, in cases where a recess is formed in an areaof a GaN material having a dislocation density of, for example, lessthan 1×10⁷/cm² using PEC etching through the application of etchingvoltage while carrying out UV light irradiation, the etching voltage ispreferably within the range of 0.16 V to 1.30 V (inclusive), morepreferably 0.52 V to 1.15 V (inclusive). Such a standard is especiallyuseful when forming a deep recess that has a depth of, for example, 1 μmor more or, for example, 2 μm or more, where the flatness of the bottomface of the recess is prone to degradation. Meanwhile, such a standardis also useful when forming a shallow recess (having a depth of, forexample, less than 1 μm), and the use of such a standard enables theformation of a bottom face of a recess having further superior flatness.This is because a decrease in etched depth results in an increase in theflatness of a bottom face of a recess.

For the purpose of increasing flatness, it is preferred that such PECetching be carried out in the manner of an intermittent repetition of UVlight irradiation and application of etching voltage. Moreover, to makeit even more preferable, the electrolyte solution used for PEC etchingis agitated during the period in which UV light irradiation andapplication of etching voltage are stopped.

In the first experimental example, flatness of the bottom face of arecess formed using PEC etching has been subject to evaluation; however,a bottom face being formed flat means etching conditions are appropriateand also implies that side faces are formed flat as well. In otherwords, carrying out PEC etching according to the conditions describedabove achieves an increase in the internal flatness of the recessformed.

For etching conditions to achieve superior flatness, an etching rate forPEC etching may be, for example, 24.9 nm/min. An etching rate anddesired etched depth may be used to estimate an etching duration. Notethat if flatness can be disregarded, a maximum etching rate for PECetching may be raised to, for example, 175.5 nm/min.

In the first experimental example, the irradiation intensity of UV lightat the etching face is 9 mW/cm². The irradiation intensity of 50 mW/cm²or 20 mW/cm² for mask aligners, for example, is a value commonly andwidely employed for UV light irradiation intensity. The firstexperimental example is carried out under a condition that facilitatesimplementation, where the irradiation intensity at the etching face is,for example, no more than 50 mW/cm² or no more than 20 mW/cm².

In the first experimental example, a NaOH solution having aconcentration of 0.01 M is used as the electrolyte solution; however,the concentration of the electrolyte solution may be adjusted asappropriate. For example, if the concentration is made lower than 0.01 M(for example, about 0.003 M), the etching flatness can be furtherincreased despite a decrease in the etching rate. Alternatively, theconcentration may be made higher than 0.01 M to such an extent thatappropriate etching flatness can be maintained (for example, 0.02 M orlower).

Third Embodiment

Next, a third embodiment will be described. The third embodimentfeatures an epitaxial substrate 30 including a GaN substrate 10 and anepitaxial layer 20 as an example of a GaN material 100, as illustratedin FIG. 14A. The constitution of the epitaxial layer 20 according to thethird embodiment differs from that of the epitaxial layer 20 accordingto the second embodiment in including a GaN layer 21 n to which n-typeimpurities have been added (also referred to as an “epitaxial layer 21n” below) and a GaN layer 21 p to which p-type impurities have beenadded (also referred to as an “epitaxial layer 21 p” below). For thesubstrate 10, the substrate 10 described in the first embodiment may beused preferably.

Although there are no particular limitations on the constitution of thesubstrate 10 and the epitaxial layer 20 (epitaxial layers 21 n and 21p), the following illustrates a possible example. For the substrate 10and the epitaxial layer 21 n, a constitution equivalent to that of thesubstrate 10 and the epitaxial layer 20 described in the secondembodiment may be adopted as an example. For the p-type impurities,magnesium (Mg) may be used as an example. The epitaxial layer 21 p maybe constituted by, for example, a stack including: a GaN layer, whichhas a thickness of between 300 nm and 600 nm (inclusive), and to whichMg has been added at a concentration of between 2×10¹⁷/cm³ and5×10¹⁸/cm³ (inclusive); and a GaN layer, which has a thickness ofbetween 10 nm and 50 nm (inclusive), and to which Mg has been added at aconcentration of between 1×10²⁰/cm³ and 3×10²⁰/cm³ (inclusive).

The epitaxial layer 20 (epitaxial layers 21 n and 21 p) may be grown onthe principal face 10 s of the substrate 10 by MOVPE, for example.Growth of the epitaxial layer 21 n is equivalent to the growth of theepitaxial layer 20 described in the second embodiment. The epitaxiallayer 21 p is grown using TMG as an example of Ga raw material, NH₃ asan example of N raw material, and Bis-cyclopentadienyl magnesium (CP₂Mg)as an example of Mg raw material. The epitaxial layers 21 n and 21 pgrow incorporating the crystallinity of the substrate 10, so similarlyto the substrate 10, have the maximum dislocation density thereoflimited to be lower than 1×10⁷/cm² while also having high in-planeuniformity. The epitaxial substrate 30 constituting the GaN material 100according to the third embodiment is a suitable material to be processedusing PEC etching to form a recess with superior internal flatness,similarly to the GaN material 100 according to the first and secondembodiments.

FIG. 14B illustrates PEC etching designed for the epitaxial substrate30. A principal face 20 s of the epitaxial layer 20 is used as anetching face. The epitaxial substrate 30 is disposed in theelectrochemical cell 300 in such a way that an area 22 to be etched inthe principal face 20 s contacts the electrolyte solution 320. Then, PECetching is carried out by applying etching voltage onto the area 22while irradiating the same with UV light 371. In this example, a recess40 is formed by penetrating through the epitaxial layer 21 p andpenetrating partway through the thickness of the epitaxial layer 21 n. Apn junction 23 pn constituted by the epitaxial layer 21 p and theepitaxial layer 21 n is exposed on a side face 23 of the recess 40. Bysetting the etching voltage to about 1 V as described above, a pnjunction 23 pn can be formed on a side face 23 having superior flatness.An area in the principal face 20 s, which is located on the outside ofthe area 22 and is not subject to etching, may be covered with a mask 41constituted by a hardmask or the like so as to be prevented from beingetched. For the purpose of limiting unwanted etching (side etching) onthe side face of the recess 40, the mask 41 may be constituted by alight-blocking mask and the linearity of the UV light 371 may beenhanced.

In cases where the epitaxial layer 20 includes an epitaxial layer 21 pto which p-type impurities have been added as in the third embodiment,it is preferred that activation annealing for activating the p-typeimpurities in the epitaxial layer 21 p be carried out after PEC etchingfor the following reason. When the epitaxial layer 21 p is a p-typeconductive layer, the epitaxial layer 21 p itself has a hole, so PECetching progresses more readily even without the UV light 371. As aresult, a difference occurs between the epitaxial layer 21 n and theepitaxial layer 21 p in terms of the ease with which etching progresses.Furthermore, since side etching occurs more readily in the epitaxiallayer 21 p, the flatness of the side face 23 of the recess 40 is proneto degradation. For this reason, it is preferred that PEC etching becarried out before the epitaxial layer 21 p is made into a p-typeconductive layer, i.e. before subjecting the epitaxial layer 21 p toactivation annealing, from the viewpoint of improving flatness. In otherwords, it is preferred that activation annealing be carried out afterPEC etching and that the epitaxial layer 21 p when subjected to PECetching has not undergone activation annealing.

FIG. 14C illustrates activation annealing. Activation annealing iscarried out to activate the n-type impurities in the epitaxial layer 21n, thereby making the epitaxial layer 21 n into an n-type conductivelayer, and to activate the p-type impurities in the epitaxial layer 21p, thereby making the epitaxial layer 21 p into a p-type conductivelayer. A technique known in the art may be used, as appropriate, tocarry out the activation annealing.

Fourth Embodiment

Next, a fourth embodiment will be described. The fourth embodiment willbe described along a second experimental example. The fourth embodimentfeatures, as an example of a GaN material 100, an epitaxial substrate 30that includes a GaN substrate 10 and an epitaxial layer 20 including anepitaxial layer 21 n to which n-type impurities have been added and anepitaxial layer 21 p to which p-type impurities have been added (seeFIG. 14A). For the substrate 10, the substrate 10 described in the firstembodiment may be used preferably.

The preferable etching conditions discussed in the first experimentalexample were applied in the second experimental example to carry out PECetching to form a cylindrical protrusion in the GaN material 100. Inother words, PEC etching was carried out to remove GaN on the outside ofthe cylindrical protrusion, the remnant serving as the cylindricalprotrusion.

The epitaxial layer 21 n was formed by growing a GaN layer having a Siconcentration of 2×10¹⁸/cm³ and a thickness of 2 μm and a GaN layerhaving a Si concentration of 2×10¹⁶/cm³ and a thickness of 10 μm on thesubstrate 10 by MOVPE. The epitaxial layer 21 p was formed by growing aGaN layer having a Mg concentration of 5×10¹⁸/cm³ and a thickness of 500nm and a GaN layer having a Mg concentration of 2×10²⁰/cm³ and athickness of 20 nm on the epitaxial layer 21 n by MOVPE. A circular maskhaving a diameter of 90 μm was formed on the epitaxial layer 21 p usinga Ti layer having a thickness of 50 nm. This mask was used in carryingout PEC etching on the GaN material 100 to a depth of 20 μm or more soas to form the cylindrical protrusion.

In this experimental example, PEC etching was carried out after carryingout activation annealing on the epitaxial layer 21 p. This was done soto confirm the degree to which side etching occurs in a condition inwhich the epitaxial layer 21 p is prone to being side etched. Thisactivation annealing was carried out in the manner of heating that wascarried out in a N₂ gas at 850° C. for 30 minutes.

FIG. 15A is a SEM image illustrating an overhead view of the cylindricalprotrusion. FIG. 15B is a SEM image illustrating a side view of thecylindrical protrusion. FIG. 16 is a SEM image in which the mask, the pnjunction, and the vicinity thereof are illustrated in enlarged fashion.These SEM images show that the PEC etching according to this embodimentenables accurate production of a structure having a desired shape thataccords with the mask.

Although etching was carried out to a depth of 20 μm or more, there isalmost no reduction in the 50 nm-thickness Ti mask. In view of this, theetching selection ratio is estimated to be at least 400 (=20 μm/50 nm).Moreover, the side etching width in the epitaxial layer 21 p directlyunder the Ti mask is estimated to be 516 nm, i.e. the width is limitedto be no more than 1 μm. A metallic material as an example of a materialthat blocks UV light may be used preferably as a mask material for PECetching, a more specific preferred example thereof to be used includingTi, Chromium (Cr), etc. The use of a mask material that limits PECetching results in a reduction in the mask thickness, and the maskthickness may be 200 nm or less, for example.

The side face of each cylindrical protrusion is roughly perpendicular tothe upper face of the cylindrical protrusion and assume the shape of asmooth curved face on the side face of the cylinder. The side faces ofthe cylindrical protrusions are homogeneous in the circumferentialdirection. The bottom face on the outside of the cylindricalprotrusions, i.e. the bottom face formed using PEC etching, is veryflat.

When observed more closely, in the side face, there are formedstripe-like fine protrusions and recesses that extend in the thickness(height) direction of the cylindrical protrusion, i.e. the direction inwhich PEC etching progresses. Such stripe-like protrusions and recessesare also homogeneous in the circumferential direction, and no anisotropyis observed in particular.

The side face is a face that is roughly perpendicular to the c face ofthe GaN crystal. The inventors of the present invention have arrived atthe finding that when a face that is perpendicular to the c face of aGaN crystal is etched with hot phosphoric acid sulfuric acid, whichconsist of the mixture phosphoric acid and sulfuric acid, the a face isetched more readily while the m face is etched less readily, andtherefore the m face is prone to being exposed. FIG. 23 is an opticalmicroscopic photograph illustrating the result of an experiment in whicha rectangular GaN material, the side faces of which are constituted byan a face and an m face perpendicular to the a face, is etched with hotphosphoric acid sulfuric acid. The right part in FIG. 23 illustratesetching results for the a face and the upper left part of FIG. 23illustrates etching results for the m face. As can be seen in thefigure, assuming that the side face of the cylindrical protrusion is aface that is formed by etching with hot phosphoric acid sulfuric acid,the portion of the side face that is perpendicular to the a axisdirection is formed from a face in which a plurality of m faces areconnected to one another in zigzag fashion so that the face isperpendicular to the a axis direction on average, and the portion of theside face perpendicular to the m axis direction is formed flat from asingle m face. In other words, when viewed in the circumferentialdirection, the constitution of a portion of such a side face that isperpendicular to the a axis direction and the constitution of a portionthereof that is perpendicular to the m axis direction demonstratedifferent anisotropies, and in the circumferential direction, overall,such a side face constitutes a coarse (angular) face in which the mfaces connect to one another in zigzag fashion. The side face of thecylindrical protrusion according to this experimental example haslimited anisotropy in the circumferential direction and is less coarse(smoother) compared to a side face that is formed by etching with hotphosphoric acid sulfuric acid. It should be noted that suchcharacteristics are also achieved for the side face of a protrusionhaving other shapes than a cylindrical shape, such as a prismaticcolumnar shape.

As can be seen from FIG. 16, the side etching width is smaller at the pnjunction, and a shape is formed such that the pn junction protrudesoutward. This reflects the fact that the hole lifetime is shorter andPEC etching progresses with more difficulty at the pn junction. Thisexperimental example illustrates an example case where PEC etching wasis carried out after the epitaxial layer 21 p has undergone activationannealing. By carrying out PEC etching before subjecting the epitaxiallayer 21 p to activation annealing, it is possible to reduce the sideetching width in the epitaxial layer 21 p, and protrusion of the pnjunction can therefore be limited.

Fifth Embodiment

Next, a fifth embodiment will be described. The fifth embodiment will bedescribed along a third experimental example. The fifth embodimentfeatures an epitaxial substrate 30, which includes a GaN substrate 10and an epitaxial layer 20 to which n-type impurities have been added, asan example of a GaN material 100 (see FIG. 3). For the substrate 10, thesubstrate 10 described in the first embodiment may be used preferably.The preferable etching conditions discussed in the first experimentalexample were applied in the third experimental example to carry out PECetching to form a cylindrical recess in the GaN material 100.

The epitaxial layer 20 was formed by growing a GaN layer having a Siconcentration of 2×10¹⁸/cm³ and a thickness of 2 μm and a GaN layerhaving a Si concentration of 1.5×10¹⁶/cm³ and a thickness of 5.8 μm onthe substrate 10 by MOVPE. A mask having circular apertures withdiameters of 1 μm, 5 μm, 10 μm, and 20 μm were formed on the epitaxiallayer 20 using a Ti layer having a thickness of 50 nm. This mask wasused in carrying out PEC etching on the GaN material 100 to a depth of7.7 μm so as to form a cylindrical recess.

FIG. 17 is a SEM image illustrating an overhead view of the cylindricalrecess. In the side face and the bottom face of the recess formed usingPEC etching, characteristics similar to those of the protrusiondescribed in the second experimental example are observed. That is, theside face of each recess is roughly perpendicular to the upper face onthe outside of the recess and assumes the shape of a smooth curved faceon the side face of the cylinder. The side faces of the recesses arehomogeneous in the circumferential direction. The bottom face of eachrecess is formed very flat. When observed more closely, in the sidefaces, stripe-like fine protrusions and recesses that extend in thethickness (depth) direction of the recesses are formed homogeneously inthe circumferential direction. The side faces have limited anisotropy inthe circumferential direction and are less coarse (smoother) compared toa side face that is formed by etching with hot phosphoric acid sulfuricacid.

In the SEM image in FIG. 17, small bumps distributed discretely can beobserved in the bottom faces of the 20 μm-diameter recesses. The areasbetween the bumps are constituted by faces that are more flat than thebumps. The bottom faces of the recesses are faces that follow the c facemore or less. The inventors of the present invention have confirmed thatdark spots are observed in the center of the bumps in SEMcathodoluminescence images. In view of this, it is considered that thebumps correspond to dislocations and since the hole lifetime is shorterand PEC etching progresses with more difficulty at the dislocations, thebumps were formed. It should be noted that a bottom face on the outsideof a protrusion in the case where a protrusion is formed as in thesecond experimental example is a bottom face that is formed using PECetching, so may have similar characteristics.

As described above, the PEC etching according to this embodiment is aprocessing method that causes almost no damage to a GaN crystal (seeFIG. 13), so damage to a side face and a bottom face formed using thePEC etching according to this embodiment is limited. Thus, in SEMcathodoluminescence images, compared to the dark spots resulting fromdislocations and observed in the bottom face, areas on the outside ofthe dislocations in the bottom faces are observed as being lighter andthe side faces are observed as being lighter compared to the dark spots.

The band-edge peak intensities of the PL emission spectra for the sideface and bottom face formed using the PEC etching according to thisembodiment have intensities that are 90% or more in relation to theband-edge peak intensity of the PL emission spectrum for a surface (anupper face on the outside of a recess when a recess is formed, or anupper face of a protrusion when a protrusion is formed) that isprotected by a mask and not etched.

Sixth Embodiment

Next, a sixth embodiment will be described. The sixth embodiment will bedescribed along a fourth experimental example. The sixth embodimentfeatures an epitaxial substrate 30, which includes a GaN substrate 10and an epitaxial layer 20 to which n-type impurities have been added, asan example of a GaN material 100 (see FIG. 3). For the substrate 10, thesubstrate 10 described in the first embodiment may be used preferably.The preferable etching conditions discussed in the first experimentalexample were applied in the fourth experimental example to carry out PECetching to form a groove-like recess (trench) in the GaN material 100.

The constitution of the epitaxial layer 20 is equivalent to thatdescribed in the third experimental example (fifth embodiment). A maskhaving linear apertures with widths of 1.4 μm, 2.8 μm, and 5.6 μm wereformed on the epitaxial layer 20 using a Ti layer having a thickness of50 nm. Two experiments were conducted using the mask: an experiment inwhich the GaN material 100 was subject to PEC etching to a target depthof 7.7 μm to form a trench; and an experiment in which the GaN material100 was subject to PEC etching to a target depth of 33 μm to form atrench.

FIGS. 18A through 18C are SEM images illustrating cross-sections oftrenches (target depth: 7.7 μm), where the cross-sections are takenalong a direction perpendicular to the length direction of the trenches.The length direction of the trench, i.e. the length direction of themask aperture, is parallel to the m axis, and the cross-section is acleavage surface of them face. FIG. 18A is a SEM image of a trenchhaving a mask aperture width of 1.4 μm, FIG. 18B is a SEM image of atrench having a mask aperture width of 2.8 μm, and FIG. 18C is a SEMimage of a trench having a mask aperture width of 5.6 μm.

The actual measured values of the depths of the trenches having the maskaperture widths of 1.4 μm, 2.8 μm, and 5.6 μm are 7.55 μm, 7.85 μm, and7.65 μm, respectively, and are all roughly equal to the target depth 7.7μm. These results show that in this experimental example, a constantetching rate can be maintained during etching if the target depth of atrench is about 8 μm, for example, and that etched depth can becontrolled on the basis of etching duration with ease.

FIGS. 19A through 19C are SEM images illustrating cross-sections oftrenches (target depth: 33 μm), where the cross-sections are taken alonga direction perpendicular to the length direction of the trenches.Similarly to FIGS. 18A to 18C, the cross-sections are each a cleavagesurface of the m face. FIG. 19A is a SEM image of a trench having a maskaperture width of 1.4 μm, FIG. 19B is a SEM image of a trench having amask aperture width of 2.8 μm and FIG. 19C is a SEM image of a trenchhaving a mask aperture width of 5.6 μm.

The depth of the trench having the mask aperture width of 5.6 μm is 32.9μm and is roughly equal to the target depth 33 μm. Meanwhile, the depthsof the trenches having the mask aperture widths of 2.8 μm and 1.4 μm are28.6 μm and 24.3 μm, respectively, and are shallower than the targetdepth 33 μm. The tendency of the trench depth being shallower than thetarget depth is more prominent as the mask aperture width is narrower.The reason therefor is considered to be that when the target depth ofthe trench is as deep as, for example, 30 μm, the influence of thearrival of UV light at the vicinity of the trench bottom being impededdue to the reduction in the mask aperture width becomes greater, leadingto a reduction in the etching rate. In this regard, an improvement in UVlight collimation is considered to limit such a reduction in the etchingrate.

As can be seen in the SEM image in FIG. 18A and the like, the edge ofthe trench aperture recedes laterally directly under the mask, or inother words, the mask protrudes in eave-like fashion on the edge of thetrench aperture, i.e. side etching is has occurred.

FIG. 20 a graph illustrating a relationship between etched depth and anaspect ratio of a trench formed in the fourth experimental example. Anaspect ratio of a recess (e.g. trench) is defined as the ratio of anetched depth (depth of the recess) W_(r) to a width W of the upper endof the recess (=W_(r)/W). The width W of the upper end of the recess isexpressed as the sum of the mask aperture width W_(mask) and a valueobtained by doubling the side etching width W_(side-etching) on one sidedirectly under the mask (=W_(mask)+2W_(side-etching)).

FIG. 20 also illustrates results for other recesses that were formedusing PEC etching while setting the mask aperture width to an equivalentvalue. The results for the trenches in the fourth experimental example(FIGS. 18A through 19C) are indicated with blacked-out plots and theresults for the other recesses are indicated with outlined plots. Thecircular plots indicate the actual measured values representing therelationships between the aspect ratios and the etched depths for themask aperture width of 1.4 μm. The square plots indicate the actualmeasured values representing the relationships between the aspect ratiosand the etched depths for the mask aperture width of 2.8 μm. Thetriangular plots indicate the actual measured values representing therelationships between the aspect ratios and the etched depths for themask aperture width of 5.6 μm. The relationships between the aspectratios and the etched depths W_(r) for the case where the mask aperturewidth W_(mask) was set to 1.4 μm, 2.8 μm, and 5.6 μm and the sideetching width W_(side-etching) on one side was assumed to be 0.7 μm areindicated by the solid line, broken line, and dotted line, respectively.

For the trench with the target depth of 7.7 μm in the fourthexperimental example: the depth and the aspect ratio for the maskaperture width of 1.4 μm are 7.55 μm and 3.1, respectively; the depthand the aspect ratio for the mask aperture width of 2.8 μm are 7.85 μmand 2.1, respectively; and the depth and the aspect ratio for the maskaperture width of 5.6 μm are 7.65 μm and 1.2, respectively. For thetrench with the target depth of 33 μm in the fourth experimentalexample: the depth and the aspect ratio for the mask aperture width of1.4 μm are 24.3 μm and 7.3, respectively; the depth and the aspect ratiofor the mask aperture width of 2.8 μm are 28.6 μm and 7.1, respectively;and the depth and the aspect ratio for the mask aperture width of 5.6 μmare 32.9 μm and 4.4, respectively

The plots of the actual measured values are more or less distributed onthe lines for the case where the side etching width is assumed to be 0.7μm. This shows that the side etching width is maintained to be constantand about 0.7 μm even when the etched depth is deepened (to be, forexample, 5 μm or more or, for example, 10 μm or more), i.e. the sideetching width is limited to be no more than 1 μm. It is also shown thatthe side etching width is maintained to be constant and about 0.7 μmeven when the mask aperture width changes.

A comparison between FIGS. 18A through 18C with FIGS. 19A through 19Creveals the tendency of the inclination of the trench side faceresulting from side etching increasing in conformity with a decrease inetched depth. In other words, it can be said that from the viewpoint ofincreasing the parallelness of side faces that face other, it ispreferable to deepen the etched depth. The following gives roughestimated values for when the side etching width directly under the maskis set to be 0.7 μm as described above and the trench bottom width andthe mask aperture width are assumed to be equivalent. It is preferredthat: the etched depth be set to 8 μm or more so as to make theinclination of the side face relative to the side etching width no morethan 5°; the etched depth be set to 10 μm or more so as to make theinclination of the side face relative to the side etching width no morethan 4°; the etched depth be set to 13.5 μm or more so as to make theinclination of the side face relative to the side etching width no morethan 3°; and the etched depth be set to 20 μm or more so as to make theinclination of the side face relative to the side etching width no morethan 2°. The side faces may be treated with tetramethylammoniumhydroxide (TMAH) and the like after PEC etching so as to make the sidefaces perpendicular.

In the state of the art, attempts are made to form trenches bysubjecting GaN to dry etching. For trenches obtained in the state of theart, however, the aspect ratio is at most about 3 and the depth is atmost about 3 μm. With dry etching, it is difficult to increase theetching selection ratio of GaN relative to the mask material, so it isnot possible to form deep trenches having a depth of, for example, 5 μmor more. Since deep trenches cannot be formed in the state of the art,trenches with a high aspect ratio of, for example, 5 or more have notbeen obtained. Moreover, dry etching has the problem of causingsignificant damage to the surface of GaN subject to etching.

The inventors of the present invention have succeeded in forming atrench having a depth of 5 μm or more in the GaN material 100 by usingthe PEC etching according to this embodiment in the fourth experimentalexample. Furthermore, the inventors of the present invention havesucceeded in forming a trench with a high aspect ratio of 5 or more.Specifically, a trench having a depth of 24.3 μm and an aspect ratio of7.3 (mask aperture width=1.4 μm; see FIG. 19A) and a trench having adepth of 28.6 μm and an aspect ratio of 7.1 (mask aperture width=2.8 μm;see FIG. 19B) could be formed.

PEC etching gives rise to side etching. If the side etching width widensin conformity with the etched depth, an increase in the aspect ratiobecomes difficult. In the fourth experimental example, the inventors ofthe present invention arrived at the undiscovered finding that using thePEC etching according to this embodiment, the side etching width can bemaintained to be more or less constant even if the etched depth deepens(to, for example, 5 μm or more or, for example, 10 μm or more). Then,based on this finding that the side etching width can be maintained tobe more or less constant, the inventors of the present invention arrivedat the undiscovered finding that the aspect ratio can be increasedproportionally to the etched depth and that it is possible to form atrench with a high aspect ratio of, for example, 5 or more (see FIG.20). According to this finding, it is also possible to form a trenchwith an aspect ratio of, for example, 10 or more.

Notwithstanding the above, the inventors of the present invention alsoarrived at the undiscovered finding mentioned above, namely thatdeepening the etched depth becomes more difficult as the mask aperturewidth narrows (see FIGS. 19A and 19B). To give a specific example, thedifficulty of deepening the etched depth increases from the maskaperture width of 5.6 μm (see FIG. 19C) to the mask aperture width of2.8 μm (see FIG. 19B). The actual trench width that is obtained byadding up the side etching widths (1.4 μm) on both sides is 7 μm for themask aperture width of 5.6 μm and 4.2 μm for the mask aperture width of2.8 μm. Thus, it can be said that when the trench width is, for example,6 μm or less or, for example, 5 μm or less, then it is difficult to forma deep trench and increase the aspect ratio. According to thisexperimental example, it was found that a trench with an aspect ratioof, for example, 5 or more can be formed even when the trench width is,for example, 6 μm or less or, for example, 5 μm or less.

Note that while a groove-like recess (trench) has been given as anexample to describe the formation of a recess with a high aspect ratio,the shape of the flat surface of the upper end (shape of the aperture)of the recess may be changed, as appropriate, and the PEC etchingaccording to this embodiment may be used to form a recess other than agroove-like recess (trench), for example, a cylindrical recess or aprismatic columnar recess with a high aspect ratio of, for example, 5 ormore or, for example, 10 or more. The width of the upper end of therecess used to define the aspect ratio may be, for example, the width ofthe shortest portion.

Although there are no particular limitations on the upper limit of theaspect ratio, the upper limit of the etched depth is the thickness ofthe GaN material 100. A recess formed using the PEC etching according tothis embodiment may encompass a structure in which the GaN material 100is penetrated through (i.e. the recess does not have a bottom face), inwhich case the etched depth of the recess penetrating through the GaNmaterial 100 will coincide with the thickness of the GaN material 100.The purpose of penetrating through the GaN material 100 may be, forexample, to form a through-hole, or to divide (separate) the GaNmaterial 100 into a plurality of segments. The aspect ratio of thethrough-hole is defined similarly to that of a recess having a bottomface, with the thickness of the GaN material serving as the etcheddepth.

In the state of the art, neither are formed protrusions (for example,ridges) with a height of 5 μm or more, nor are formed protrusions (forexample, ridges) with a high aspect ratio of 5 or more. The PEC etchingaccording to this embodiment can also be used to form a protrusionhaving a height of 5 μm or more and to form a protrusion with an aspectratio of, for example, 5 or more or, for example, 10 or more (see FIG.22B). An aspect ratio of a protrusion (e.g. ridge) is defined as theratio of an etched depth (height of the protrusion) W_(r) to a width Wof the upper end of the protrusion (=W_(r)/W). The width W of the upperend of the protrusion is expressed as a difference between the maskwidth W_(mask) and a value obtained by doubling the side etching widthW_(side-etching) on one side directly under the mask(=W_(mask)−2W_(side-etching)). The shape of the flat surface of theupper end of the protrusion may be selected, as appropriate. The widthof the upper end of the protrusion used to define the aspect ratio maybe, for example, the width of the shortest portion.

In conformity with the above discussion about trench formation, alsowhen forming a protrusion, the side etching width directly under themask relative to the PEC etching according to this embodiment ismaintained to be constant and about 0.7 i.e. limited to be no more than1 more or less independently of the etched depth or the mask width.Conditions for limiting the inclination of a side face relative to theside etching width are equivalent to those that have been discussed inrelation to trench formation.

It should be noted that the PEC etching according to this embodiment mayalso be used preferably when forming a recess or a protrusion with anaspect ratio of, for example, less than 5. As described above, the PECetching according to this embodiment is a processing method that causesalmost no damage to a GaN crystal, so this PEC etching is favorable forthe formation of any recesses or protrusions regardless of the aspectratio.

Seventh Embodiment

Next, a seventh embodiment will be described. The seventh embodimentdescribes a technique for performing PEC etching on the entirety of theetching face of the GaN material 100 having a large diameter of, forexample, two inches or more (PEC etching that can be suitably performedon an intended area within the entire etching face).

With the PEC etching (see FIG. 4 and (chem. 1) through (chem. 4))described in the second embodiment (first experimental example),electrons generated together with holes in the anode electrode 340 (GaNmaterial 100) due to irradiation with the UV light 371 are supplied tothe cathode electrode 330 via the wire 350 and are thus consumed bycathode reaction (chem. 4). The wire 350 is attached to the anodeelectrode 340 (GaN material 100) via an ohmic contact probe 341 arrangedso as not to contact the electrolyte solution 320 (so as not to causeshort circuiting). Such PEC etching requires a sealing structure forpreventing contact between the ohmic contact probe 341 and theelectrolyte solution 320.

It is troublesome to form a sealing structure for the entirety of theetching face of the GaN material 100 with a large diameter, so it ispreferable to omit such a sealing structure. Thus, in this embodiment,there will be considered PEC etching in which the ohmic contact probe341, wire 350, and cathode electrode 330 are omitted. PEC etching inwhich the ohmic contact probe 341, wire 350, and cathode electrode 330are omitted may be referred to as “electrodeless PEC etching” below,whereas PEC etching in which the ohmic contact probe 341, wire 350, andcathode electrode 330 are used may be referred to as “electrode-basedPEC etching” below.

FIGS. 24A through 24D are schematic diagrams each illustrating anexample of a manner in which electrodeless PEC etching according to theseventh embodiment is carried out. FIG. 24A will be referenced. The GaNmaterial 100 (may be referred to as “wafer 100” below) is prepared. Thewafer 100 has a large diameter (large area) of, for example, two inchesor more. The structure of the wafer 100 may be selected, as appropriate.The wafer 100 may be a substrate itself, such as that illustrated inFIG. 1G, or may be a stack formed from a substrate and anepitaxially-grown layer, such as that illustrated in FIG. 3 or FIG. 14A,for example. A plurality of etching areas 111 through 113 disposed atintervals from one another are defined on a surface 100 s (etching face)of the wafer 100.

The etching areas 111 through 113 are areas where the GaN material 100is to be etched by being irradiated with UV light 431 while beingimmersed in (being in contact with) an etching liquid 420, as will bedescribed later. A light-blocking mask 41 for blocking the UV light 431is formed on the surface 100 s of the wafer 100. The mask 41 has anopening for exposing the etching areas 111 through 113. The currentexample illustrates three etching areas 111 through 113, but the numberof etching areas may be modified according to need, as appropriate. Theshape and size of each of the etching areas 111 and the like, therespective positions of the etching areas 111 and the like on thesurface 100 s, etc. may be modified according to need, as appropriate.

FIG. 24B will be referenced. The etching liquid 420 is prepared in acontainer 410. The etching liquid 420 contains an electrolyte solutionequivalent to the electrolyte solution 320 described in the secondembodiment (first experimental example) and potassium peroxodisulfate(K₂S₂O₈) dissolved in the electrolyte solution. The electrolyte solutionmay, for example, be a NaOH solution, as descried above. The etchingliquid 420 used for the electrodeless PEC etching in this embodimentcontains a hydroxide ion (OH⁻) and a peroxodisulfate ion (S₂O₈ ²⁻).Meanwhile, the etching liquid (the electrolyte solution 320) used forthe electrode-based PEC etching in the second embodiment contains ahydroxide ion (OH⁻) but does not contain a peroxodisulfate ion (S₂O₈²⁻).

FIG. 24C will be referenced. The wafer 100 on which the mask 41 isformed is accommodated in the container 410 in a condition in which thesurface 100 s of the wafer 100 is immersed in the etching liquid 420 ina manner such that the surface 100 s is parallel with a surface 420 s ofthe etching liquid 420 (so as to be horizontal). With the electrodelessPEC etching of this embodiment, the entirety of the wafer 100 may beimmersed in the etching liquid 420, so etching can be performed withoutproviding a sealing structure such as that described above. The surface100 s of the wafer 100 and the surface 420 s of the etching liquid 420being “parallel” with each other means that the angle between thesurface 100 s of the wafer 100 and the surface 420 s of the etchingliquid 420 is within the range of 0°±2°.

Note that the wafer 100 and the etching liquid 420 may be accommodatedin the container 410 in the order of first accommodating the etchingliquid 420 in the container 410 and then accommodating the wafer 100 inthe container 410, or in the order of first accommodating the wafer 100in the container 410 and then accommodating the etching liquid 420 inthe container 410. The process of preparing the etching liquid 420 inthe container 410 and the process of accommodating the wafer 100 in thecontainer 410 may be performed with either one of these orders accordingto need.

FIG. 24D will be referenced. In a condition in which the wafer 100 andthe etching liquid 420 are still, the UV light 431 is radiated from alight irradiation device 430 onto the surface 100 s of the wafer 100. UVlight with a wavelength of shorter than 365 nm is used for the UV light431. The UV light 431 is radiated on the respective surfaces of theetching areas 111 through 113 perpendicularly from the surface 420 sside of the etching liquid 420, i.e. from the upper side.“Perpendicular” to the surfaces of the etching areas 111 through 113,i.e. to the surface 100 s of the wafer 100, means that the angle of theUV light 431 with respect to the surface 100 s of the wafer 100 iswithin the range of 90°±2°.

The concept of “radiating the UV light 431 onto the surface 100 s of thewafer 100” or “irradiating the surface 100 s of the wafer 100 with theUV light 431” means “radiating the UV light 431 toward the surface 100 sof the wafer 100”, and likewise, the concept of “radiating the UV light431 onto the wafer 100” or “irradiating the wafer 100 with the UV light431” means “radiating the UV light 431 toward the wafer 100”. The UVlight 431 radiated onto the mask 41 is equivalent to same being radiatedtoward the surface 100 s of the wafer 100 or same being radiated towardthe wafer 100.

With the electrodeless PEC etching in this embodiment, the reactions(chem. 5) through (chem. 7) below occur, and the cathode reaction (chem.4) in the electrode-based PEC etching of the second embodiment isreplaced by (chem. 7) so that the electrons generated together with theholes in the GaN material 100 are consumed. Thus, the cathode electrode330 used in the electrode-based PEC etching of the second embodiment canbe omitted, and accordingly, the wire 350 and the ohmic contact probe341 can also be omitted. In other words, PEC etching is performedwithout using the cathode electrode 330 that is immersed in the etchingliquid 420 while being connected to the wire 350 extending to outsidethe etching liquid 420 (without using the cathode electrode 330 immersedin the etching liquid 420 and the wire 330 connected to the cathodeelectrode 330 and extending to outside the etching liquid 420). Theanode reactions (chem. 1) through (chem. 3) in the GaN material 100 arethe same for both the electrodeless PEC etching and the electrode-basedPEC etching. In this way, the GaN material 100 can be etched using theelectrodeless PEC etching of this embodiment.K₂S₂O₈→2K⁺+S₂O₈ ²⁻  [Chemical Formula 5]S₂O₈ ²⁻+heat or hv→2SO₄ ⁻*  [Chemical Formula 6]2SO₄ ⁻*+2e ⁻+2H₂O(I)→2SO₄ ²⁻+2HO*+H₂(g)↑  [Chemical Formula 7]

The etching areas 111 through 113 are located in dispersed fashion onthe surface 100 s of the wafer 100. An increase in the diameter of thewafer 100 results in enlargement in the overall range in which theetching areas 111 through 113 are located. Due to enlargement in theoverall range in which the etching areas 111 through 113 are located,uniform etching of the etching areas 111 through 113 becomes difficult.In this embodiment, the uniformity of etching of the etching areas 111through 113 can be enhanced in the manner described below.

The wafer 100 is accommodated in the container 410 such that the surface100 s thereof is parallel with the surface 420 s of the etching liquid420. In this way, for all of the etching areas 111 through 113, thedistance (depth) from the surface 420 s of the etching liquid 420, i.e.the thickness of the etching liquid 420 located above the etching areas111 through 113, can be made uniform. As a result, it is possible tolimit variation in the condition in which (at least one from among) OH⁻and S₂O₈ ²⁻ (or SO₄ ⁻*) are (is) supplied to the etching areas 111through 113 as a result of diffusion from the etching liquid 420 locatedabove the etching areas 111 through 113; therefore, uniformity ofetching in the surface 100 s of the wafer 100 can be enhanced. Moreover,advantages of making the surface 100 s of the wafer 100 and the surface420 s of the etching liquid 420 also include achievement of uniformityin the supply of SO₄ ⁻* due to absorption of the UV light 431 in (chem.6).

A distance L from the surface 100 s of the wafer 100 to the surface 420s of the etching liquid 420 may preferably be between 1 mm and 100 mm(inclusive), for example. When the distance L is too short, variation inthe condition in which (at least one from among) OH⁻ and S₂O₈ ²⁻ (or SO₄⁻*) are (is) supplied to the etching areas 111 through 113 as a resultof diffusion from the etching liquid 420 located above the etching areas111 through 113 increases easily. Thus, it is preferable to set thedistance L to 1 mm or more, for example. When the distance L is toolong, the container 410 will be too large, and the amount of etchingliquid 420 will also be too large. Thus, it is preferable to set thedistance L to 100 mm or less, for example. In addition, from theperspective of supplying SO₄ ⁻*, when the distance L is too short,absorption of the UV light 431 on the surface 100 s of the wafer 100will be predominant and there will be an excess in electrons. Meanwhile,when the distance L is too long, most of the UV light 431 will beabsorbed by the reaction of (chem. 6) in the etching liquid 420 so thatsufficient holes cannot be supplied to the wafer 100 and thus the anodicoxidation of the reaction of (chem. 1) cannot be performed. As such, itis preferable to adjust the distance L so that there is an amount of SO₄⁻* whereby holes can be supplied to the wafer 100 while excessiveelectrons can be eliminated in the etching liquid 420. Also from thisperspective, it is preferable to set the distance L to between 1 mm and100 mm (inclusive), for example. When the distance L is short, it willbe difficult to control the height of the liquid surface, so thedistance L is preferably 1 mm or more, more preferably 3 mm or more,even more preferably 5 mm or more. Since the mask 41 has a smallthickness, such as 200 nm or less as exemplified above, the distance Lfrom the surface 100 s of the wafer 100 to the surface 420 s of theetching liquid 420 can be regarded as the distance from the surface ofthe mask 41 to the surface 420 s of the etching liquid 420.

The UV light 431 is radiated in a condition in which the wafer 100 andthe etching liquid 420 are still. As a result, it is possible to limitvariation in the condition in which (at least one from among) OH⁻ andS₂O₈ ²⁻ (or SO₄ ⁻*) are (is) supplied to the etching areas 111 through113 due to movement of the etching liquid 420; therefore, uniformity ofetching in the surface 100 s of the wafer 100 can be enhanced.

The UV light 431 is radiated on the respective surfaces of the etchingareas 111 through 113 perpendicularly. As a result, it is possible toperpendicularly align the depth directions of recesses 121 through 123formed respectively in the etching areas 111 through 113; therefore,uniformity of etching in the surface 100 s of the wafer 100 can beenhanced.

Now, FIGS. 18A through 18C and FIGS. 19A through 19C of the sixthembodiment (fourth experimental example) will be referenced. Acomparison of the shallow trench illustrated in FIGS. 18A through 18Cand the deep trench illustrated in FIGS. 19A through 19C reveals that inthe deep trench illustrated in FIGS. 19A through 19C, deviation(inclination) of the depth direction from the perpendicular direction isrelatively more conspicuous. Such inclination is less conspicuous in theshallow trench illustrated in FIGS. 19A through 19C. As such, to presenta rough estimate, it is preferable to perform perpendicular irradiationsuch as that of this embodiment in cases where the depth of the recessformed is 8 μm or more, for example, or 10 μm or more, for example.

From the perspective of, for example, radiating light efficiently to adeep position in each of the recesses 121 through 123 formed, it ispreferable that the UV light 431 radiated onto each of the etching areas111 through 131 be parallel light in which the directions of all raysare aligned perpendicularly; however, it is allowable to adopt lightother than parallel light (convergent light or diffused light). Theconcept of being “radiated perpendicularly” means that in the UV light431 radiated onto each of the etching areas 111 through 113, thecomponent radiated perpendicularly has the greatest intensity.

In order for etching conditions for the etching areas 111 through 113 tobe uniform, it is preferred that the irradiation intensity of the UVlight 431 in the etching areas 111 through 113 be uniform. Moreover, inorder for etching depths for the etching areas 111 through 113 to beuniform, it is preferred that the cumulative irradiation energy of theUV light 431 in the etching areas 111 through 113 be uniform. Theirradiation intensity is expressed in unit W/cm² and the irradiationenergy is expressed in unit J/cm². The cumulative irradiation energymeans the total amount of irradiation energy of the UV light 431radiated up to the completion of the PEC etching in the etching areas111 through 113 (completion of the formation of the recesses 121 through123). The irradiation intensity and irradiation energy being “uniform”means that variation in the ratio of a maximum value to a minimum valueis small, such as preferably 130% or less, more preferably 120% or less,even more preferably 110% or less.

The PEC etching method of this embodiment may be used preferably as astructure production method for forming structures on the wafer 100 byPEC etching. A PEC etching apparatus 400 constituting a structureproduction apparatus used for such a structure production methodincludes the container 410, the light irradiation device 430, and acontrol device 440, as illustrated in FIG. 24D. The control device 440includes: a storage device storing, for example, data and a program forcontrolling the light irradiation device 430 and other such devices ofthe PEC etching apparatus 400 so that these devices perform prescribedoperations; and a CPU for reading the program and the like from thestorage device and executing same. The control device 440 may be formedfrom a personal computer, for example. A device that is a combination ofthe light irradiation device 430 and the control device 440 may beregarded as a light irradiation device.

The light irradiation device 430 includes a light source for emitting UVlight and an optical device for guiding the UV light emitted from thelight source to irradiate the wafer 100. If the UV light emitted fromthe light source is to be radiated directly onto the wafer 100, theoptical device may be omitted. For the light source, there may be used alight source which emits light including UV light with a wavelength ofshorter than 365 nm so as to enable PEC etching of GaN, and anultraviolet light emitting diode (LED), ultraviolet laser, ultravioletlamp, and the like may be used preferably therefor, for example. Theoptical device may include various optical members, as needed, so thatthe UV light 431 is radiated onto the wafer 100 under prescribedconditions. The various optical members may include: various types oflenses; various types of mirrors; an intensity distribution adjustingdevice for achieving a prescribed irradiation intensity distribution inan irradiation cross-section on the wafer 100 (the “irradiationcross-section” being a cross-section of light corresponding to theregion that is being irradiated with the light); a cross-section shapingdevice for achieving a prescribed irradiation cross-section shape; ascanning device for moving the irradiation cross-section to a prescribedlocation on the wafer 100; a collimation optical system for obtainingparallel light; a chopper for performing intermittent light radiation onthe wafer 100; a filter for adjusting the wavelength of the radiatedlight; etc. A digital micromirror device (DMD) may be used, for example,to regulate the irradiation cross-section shape of the UV light 431radiated onto the wafer 100 into a prescribed pattern.

FIGS. 25A and 25B are timing charts each illustrating an example of amanner in which PEC etching according to this embodiment is carried out,and illustrate a manner in which the UV light 431 may be radiated ontothe wafer 100. In a first example illustrated in FIG. 25A, the UV light431 is radiated onto the etching areas 111 through 113 simultaneously.For performing radiation in this manner, it is preferable to radiate,for example, UV light 431 which has an irradiation cross-section of asize encompassing all the etching areas 111 through 113 and for whichthe irradiation intensity distribution in the irradiation cross-sectionis uniform. Accordingly, the irradiation intensity of the UV light 431radiated onto the etching areas 111 through 113 can be made uniform.Also, the cumulative irradiation energy of the UV light 431 radiatedonto the etching areas 111 through 113 can be made uniform accordingly.Note that FIG. 24D illustrates a plurality of arrows so as to make thecomponents of the UV light 431 radiated on each of the etching areas 111through 113 easy to ascertain, but this does not mean that a separate UVlight 431 ray is to be radiated onto each of the etching areas 111through 113; in this example, the UV light 431 illustrated in FIG. 24Dis integrated (a single ray of) light that has an irradiationcross-section of a size encompassing all of the etching areas 111through 113.

In this example, the UV light 431 is radiated onto the etching areas 111through 113 simultaneously, so compared to the manner of radiationaccording to a second example described later, the time required tocomplete the formation of the recesses 121 through 123 can be shortened.Note that, if necessary, it is also possible to radiate separate UVlight 431 (a plurality of rays) onto the respective etching areas 111through 113 simultaneously.

It is preferable to radiate the UV light 431 onto the etching areas 111through 113 intermittently. By doing so, a process in which Ga₂O₃generated in a UV light 431 radiation period is dissolved in a UV light431 non-radiation period is repeated. In other words, a process, inwhich Ga₂O₃ is formed extremely thinly and the Ga₂O₃ formed extremelythinly is dissolved, is repeated. Accordingly, flatness of the surfaceformed by etching can be increased compared to when performing a processin which Ga₂O₃ is formed thickly through continuous radiation of the UVlight 431 and all of the thickly formed Ga₂O₃ is dissolved at once. Suchintermittent radiation may be performed by, for example, turning theswitch of the light source on and off, or by using a chopper. For theintermittent radiation, the length of the radiation period for one pulseand the length of the non-radiation period between two consecutivepulses may be set, as appropriate, through an experiment. It is possibleto, by performing intermittent radiation, consume the electronsaccumulated on the wafer 100 during the radiation period throughnonradiative recombination, or the like, using the non-radiation period.

In the second example illustrated in FIG. 25B, the UV light 431 isradiated asynchronously onto the etching areas 111 through 113. In thismanner of radiation, the irradiation cross-section of the UV light 431may be moved over, so as to scan, the surface 100 s of the wafer 100,for example. This movement is performed such that perpendicularradiation onto the surface 100 s of the wafer 100 is maintained.

In this example, even a light source with a small output, i.e. a lightsource that is incapable of providing sufficient irradiation intensityfor a wide irradiation cross-section, can be used to perform PEC etchingon the wafer 100 with a large diameter by moving a narrow irradiationcross-section with which sufficient irradiation intensity can beachieved.

As described in the first example, it is preferable to performintermittent radiation in the second example as well. Note that in thesecond example, since a non-radiation period is interposed due tomovement of the light, irradiation of the etching areas 111 through 113is intermittent.

Next, a first variation of the seventh embodiment will be described. Inthis variation, described will be a case in which two or more wafers 100are PEC etched using a single container 410. Herebelow, a specificexample will be described in which PEC etching is performed on twowafers 100, but PEC etching for three or more wafers 100 can beperformed on the basis of the same concept.

FIG. 26 is a schematic diagram illustrating an example of a manner inwhich electrodeless PEC etching according to this variation is carriedout. In this variation, the container 410 is configured to be capable ofaccommodating two or more wafers 100. Similarly to the process describedwith reference to FIG. 24A, two, first and second, wafers 100 areprepared.

Similarly to the process described with reference to FIG. 24C, the firstand second wafers 100 are accommodated in the container 410.Specifically, the first and second wafers 100 are accommodated in thecontainer 410 in a condition in which surfaces 100 s of the first andsecond wafers 100 are immersed in the etching liquid 420 in a mannersuch that the surfaces 100 s of the first and second wafers 100 are bothparallel with the surface of the etching liquid 420 and the distancefrom the surface 100 s of the first wafer 100 to the surface 420 s ofthe etching liquid 420 and the distance from the surface 100 s of thesecond wafer 100 to the surface 420 s of the etching liquid 420 are thesame distance L.

Similarly to the process described with reference to FIG. 24D, in acondition in which the first and second wafers 100 and the etchingliquid 420 are still, the UV light 431 is radiated from lightirradiation devices 430 perpendicularly onto surfaces of etching areas111 through 113 of both the first and second wafers 100. Note thatalthough FIG. 26 illustrates a configuration in which a separate lightirradiation device 430 is provided for each of the first and secondwafers 100, it is also possible to adopt a configuration in which asingle light irradiation device 430 (light irradiation device 430 aillustrated in FIG. 26) is provided for the two wafers 100.

As described in the seventh embodiment, in this variation, uniformity ofetching in the surfaces 100 s of the first and second wafer 100 can beenhanced. Moreover, in this variation, the PEC etching is performedwhile making the distance from the surface 100 s of the first wafer 100to the surface 420 s of the etching liquid 420 and the distance from thesurface 100 s of the second wafer 100 to the surface 420 s of theetching liquid 420 the same distance L. In this way, the thickness ofthe etching liquid 420 located above the etching areas 111 through 113of both wafers 100 can be made uniform. As a result, it is possible tolimit variation in the condition in which (at least one from among) OH⁻and S₂O₈ ²⁻ (or SO₄ ⁻*) are (is) supplied between the etching areas 111and the like of the first wafer 100 and the etching areas 111 and thelike of the second wafer 100; therefore, uniformity of etching betweenboth wafers 100 can be enhanced.

From the perspective of enhancing uniformity of etching, it is morepreferred that the irradiation intensity and/or cumulative irradiationenergy of the UV light 431 radiated onto the etching areas 111 through113 be the same between both wafers 100. In order to make theirradiation intensity and/or cumulative irradiation energy the samebetween both wafers 100, a single light irradiation device 430 (430 a)may be provided for both wafers 100 and UV light 431, which has anirradiation cross-section of a size encompassing all etching areas 111through 113 in both wafers 100 and has a uniform irradiation intensitydistribution in the irradiation cross-section, may be radiated, forexample. In such an example, the UV light 431 illustrated in FIG. 26,i.e. the UV light 431 radiated onto both wafers 100 from a single lightirradiation device 430 a, is integrated (a single ray of) light that hasan irradiation cross-section of a size encompassing all the etchingareas 111 through 113 of both wafers 100 (the plurality of arrows inFIG. 26 illustrate components of the integrated (single ray of) UV light431 that are radiated onto the respective etching areas 111 through 113of both wafers 100).

Next, a second variation of the seventh embodiment will be described.The feature described in the seventh embodiment may be applied toelectrode-based PEC etching. FIG. 27 is a schematic diagram illustratingan example of a manner in which electrode-based PEC etching according tothis variation is carried out. The PEC etching apparatus 400 of thisvariation is configured such that the PEC etching apparatus 400according to the seventh embodiment is further equipped with a cathodeelectrode 451, a wire 452, an ohmic contact probe 453, and a voltagesource 454. The etching liquid 420 of this variation does not haveK₂S₂O₈ added, and contains OH⁻ but does not contain S₂O₈ ²⁻.

Similarly to the second embodiment (first experimental example), thecathode electrode 451 is immersed in the etching liquid 420 and iselectrically connected to the ohmic contact probe 453 via the wire 452.The ohmic contact probe 453 is electrically connected to the wafer(anode electrode) 100 without contacting (while being electricallyinsulated from) the etching liquid 420. While being sealed from theetching liquid 420, the ohmic contact probe 453 is connected to the rearsurface of the wafer 100 from the side of the bottom face of thecontainer 410, for example. A structure enabling such connection isformed on the bottom face of the container 410. The voltage source 454applies an etching voltage under the conditions described in the secondembodiment (first experimental example) by being controlled by thecontrol device 440. As a result, it is possible to control the etchingconditions based on the etching voltage.

In this variation as well, the wafer 100 is accommodated in thecontainer 410 in a manner such that the surface 100 s of the wafer 100is parallel with the surface 420 of the etching liquid 420, and the UVlight 431 is radiated onto the etching areas 111 through 113perpendicularly in a condition in which the wafer 100 and the etchingliquid 420 are still. Accordingly, similarly to the seventh embodiment,uniformity of etching in the surface 100 s of the wafer 100 can beenhanced.

Eighth Embodiment

Next, an eighth embodiment will be described. Due to PEC etching,nitrogen gas (N₂ gas) occurs as indicated in (chem. 1), oxygen gas (O₂gas) occurs as indicated in (chem. 3), and hydrogen gas (H₂ gas) occursas indicated in (chem. 4) or (chem. 7). Thus, a bubble 130 containing atleast one from among N₂ gas, O₂ gas, and H₂ gas occurs in the etchingliquid 420. When the UV light 431 is radiated onto the etching areas 111and the like of the wafer 100 in a condition in which an excessiveamount of bubbles 130 have adhered thereon, it is difficult to properlyradiate the light onto the etching areas 111 and the like due to, forexample, the light being diffused by the bubbles 130.

Moreover, the etching liquid 420 used for PEC etching degrades due tovarious types of reactions accompanying the PEC etching. If the etchingliquid 420 degrades excessively, the PEC etching cannot proceedproperly.

The eighth embodiment describes a manner of PEC etching in which bubbles130 adhering to the wafer 100 are removed. The eighth embodiment alsodescribes a manner of PEC etching in which the etching liquid 420 isreplenished.

FIG. 28 is a schematic diagram illustrating an example of a manner inwhich PEC etching according to the eighth embodiment is carried out. Thefollowing example employs electrodeless PEC etching, but also whenemploying electrode-based PEC etching, the same concept can be used toremove the bubble 130 and replenish the etching liquid 420. The PECetching apparatus 400 of this embodiment is configured such that the PECetching apparatus 400 according to the seventh embodiment is furtherequipped with a support 411, a vibration generator 450, a pump 460, anda tank 461.

The vibration generator 450 imparts vibration to a bubble 130 to remove(detach) the bubble 130 adhering to the wafer 100 from the wafer 100. Anultrasound generator may be used as the vibration generator 450, forexample. This embodiment illustrates a configuration in which thesupport 411 for the wafer 100 is provided on the bottom face of thecontainer 410 and a vibration member (e.g. ultrasonic vibrator) of thevibration generator 450 is provided in the support 411. The vibrationgenerator 450 vibrates the rear surface of the wafer 100 and vibrationis imparted to the bubble 130 via the wafer 100. The manner ofinstalling the vibration generator 450 is not limited to theaforementioned, and the vibration generator 450 may be attached to anouter surface of the container 410, for example. When installed in thismanner, the vibration generator 450 vibrates the etching liquid 420 viathe container 410 and vibration is imparted to the bubble 130 via theetching liquid 420. Alternatively, the vibration generator 450 may beinstalled by being embedded in the container 410 itself. The vibrationgenerator 450 is an example of a bubble removal device. The vibrationgenerator 450 performs a prescribed operation by being controlled by thecontrol device 440. A device that is a combination of the vibrationgenerator 450 and the control device 440 may be regarded as a vibrationgenerator.

A flow path for sending the etching liquid 420 from the container 410toward the pump 460 side and a flow path for sending the etching liquid420 from the pump 460 side into the container 410 are connected to thecontainer 410 in this embodiment. The pump 460 enables supply anddischarge of the etching liquid 420. A new etching liquid 420accommodated in the tank 461 is supplied into the container 410 by thepump 460. In this way, the etching liquid 420 is replenished in thecontainer 410. The old etching liquid 420 discharged from the container410 as the new etching liquid 420 is supplied may be discarded(recovered). However, as long as there is no significant impact on theetching result (if the concentrations of the various components are notchanged significantly), the etching liquid 420 in the container 410 maybe increased in accordance with the supply of the new etching liquid420. The pump 460 and the tank 461 are examples of an etching liquidreplenishment device. The pump 460 performs a prescribed operation bybeing controlled by the control device 440. A device that is acombination of the pump 460 and the control device 440 may be regardedas a pump.

FIG. 29 is a timing chart illustrating an example of a manner in whichPEC etching according to the eighth embodiment is carried out, andillustrates a manner in which the UV light 431 may be radiated onto thewafer 100 and a manner in which the vibration generator 450 and the pump460 may operate.

Prior to radiation of the UV light 431 onto the wafer 100, similarly tothe seventh embodiment, the wafer 100 is prepared, the etching liquid420 is prepared in the container 410, and the wafer 100 is accommodatedin the container 410. Accordingly, the wafer 100 is disposed while thesurface 100 s of the wafer 100 is in contact with the etching liquid420.

Then, in a period T1, a process of radiating the UV light 431 onto thesurface 100 s of the wafer 100 (referred to as “irradiation process”below) is performed. This example illustrates a case where in theirradiation process, light is radiated onto the plurality of etchingareas 111 and the like simultaneously. In other words, in theirradiation process, the UV light 431 is radiated onto the wafer 100similarly to the manner described with reference to FIG. 25A in theseventh embodiment. Note, however, that the irradiation process may beperformed in a manner such that light is radiated onto the plurality ofetching areas 111 and the like asynchronously as described withreference to FIG. 25B.

After the irradiation process, in a period T2, a process of removing abubble 130 adhering to the wafer 100 from the wafer 100 by operating thevibration generator 450 (referred to as “bubble removal process” below)is carried out. Moreover, in the period T2, a process of replenishingthe etching liquid 420 in the container 410 by operating the pump 460(referred to as “replenishment process” below) is carried out.

By radiating the UV light 431 from the upper side in the irradiationprocess, the recesses 121 and the like open on the upper side areformed. Accordingly, removal of the bubble 130 from inside the recesses121 and the like by floating the bubble 130 is facilitated.

As described above, in the irradiation process, the UV light 431 isradiated in a condition in which the wafer 100 and the etching liquid420 are still. In the bubble removal process, the etching liquid 420undergoes movement due to removal of the bubble 130. In thereplenishment process, the etching liquid 420 also undergoes movementdue to replenishment of the etching liquid 420. Thus, it is preferableto carry out the bubble removal process and the replenishment process ina different period from the irradiation process. In other words, it ispreferred that the UV light 431 not be radiated on the surface 100 s ofthe wafer 100 in the period T2 in which the bubble removal processand/or the replenishment process is/are performed.

After the bubble removal process and the replenishment process, in aperiod T3, a process of waiting for the etching liquid 420 to becomestill (also referred to as “stillness awaiting process” below) iscarried out. In other words, preparation for performing anotherirradiation process in a condition in which the etching liquid 420 hasbecome still is carried out. The stillness awaiting process may becarried out by, for example, the control device 440 measuring time so asto confirm whether the period T3 determined in advance has elapsed afterthe end of the period T2. The length of the period T3 required for theetching liquid 420 to become still may be determined through anexperiment, for example.

A process constituted by a set including the irradiation process, thebubble removal process plus the replenishment process, and the stillnessawaiting process is repeated a plurality of times. The recesses 121 andthe like are deeper as the number of this repetition is larger. When therecesses 121 and the like reach a prescribed depth (i.e. when theformation of the recesses 121 and the like is complete), the PEC etchingis terminated.

In this embodiment, the irradiation process and the bubble removalprocess (or the replenishment process) are repeated a plurality of timesalternately, and the stillness awaiting process is placed between thebubble removal process (or the replenishment process) and theirradiation process performed immediately after the bubble removalprocess (or the replenishment process). It can be said that the periodin which the bubble 130 is removed from the wafer 100 in the bubbleremoval process (or the period in which the etching liquid 420 isreplenished in the replenishment process) and the period in which the UVlight 431 is radiated onto the surface 100 s of the wafer 100 in theirradiation process performed immediately after the bubble removalprocess (or the replenishment process) are non-continuous.

In the irradiation process (during the period T1), the UV light 431 isradiated intermittently. When such intermittent radiation is performed,the degree to which bubbles 130 adhere to the wafer 100 (the coveragerate of the bubbles 100 covering the wafer 100) increases as the numberof times of light radiation increases. The timing at which the period T2is started, i.e. the length of the period T1, is set so that the bubbleremoval process (period T2) starts before the adhesion of the bubbles130 becomes excessive. The timing at which the period T2 is started andthe length of the period T2 required for the bubble(s) 130 to be removedadequately may be determined through an experiment, for example.

It is possible to adopt a manner in which, for every single pulse of theintermittent radiation, the bubble 130 is removed and the period for theetching liquid 420 to become still is waited; however, with this manner,the non-radiation period between two consecutive pulses cannot be madeto be shorter than or equal to the period for removing the bubble 130and waiting until the etching liquid 420 becomes still. In thisembodiment, compared to this manner, a plurality of pulses are radiated(the intermittent radiation is performed) up to a point before adhesionof the bubbles 130 becomes excessive within a single irradiationprocess, so the non-radiation period between two consecutive pulses canbe shortened and hence PEC etching can be performed time-efficiently.

It is not always necessary to carry out both the bubble removal processand the replenishment process together in the period T2. For example,within the repetitions, the bubble removal process may be performedwithout the replenishment process in a certain period T2, then in adifferent period T2, both the bubble removal process and thereplenishment process may be performed. The length of the stillnessawaiting process (period T3) may be varied between when the process isperformed immediately after a period T2 in which the bubble removalprocess is performed without the replenishment process and when theprocess is performed immediately after a period T2 in which both thebubble removal process and the replenishment process are performed, forexample. If the time required for the etching liquid 420 to become stillis longer for the replenishment process than the bubble removal process,the stillness awaiting process (period T3) performed immediately afterthe period T2 in which the replenishment process is performed may bemade longer than the stillness awaiting process (period T3) performedimmediately after the period T2 in which the replenishment process isnot performed, for example.

Both the bubble removal process and the replenishment process areprocesses that induce movement of the etching liquid 420. Thus, if boththe bubble removal process and the replenishment process are to beperformed, from the perspective of shortening the length of thestillness awaiting process (period T3), it is preferred that the bubbleremoval process and the replenishment process be performedsimultaneously (i.e. the period for removing the bubble 130 from thewafer 100 and the period for replenishing the etching liquid 420 in thecontainer 410 overlap each other in terms of time).

As described above, according to this embodiment, through removal ofbubbles 130, it is possible to limit the occurrence of a case whereproper light radiation is difficult due to adhesion of a bubble 130 onthe wafer 100. Moreover, through replenishment of the etching liquid420, it is possible to limit the occurrence of a case where the PECetching cannot proceed properly due to degradation of the etching liquid420. In this way, according to this embodiment, PEC etching can beperformed while limiting variations in etching conditions in terms oftime.

The flow of the etching liquid 420 generated by the pump 460 may havethe action of removing the bubble 130 in the replenishment process. Useof the vibration generator 450 makes the removal of the bubble 130 morereliable. The surfactant added to the etching liquid 420 demonstrates aneffect to make the bubble 130 less adherable to the wafer 100.

Note that in the second embodiment (first experimental example), whenthe pump 380 agitates the electrolyte solution 320 in the container 310(the first five seconds of the period in which the UV light radiationand the etching voltage application are stopped (illustrated in FIG.5)), the electrolyte solution (etching liquid) 320 is replenished.Moreover, the agitation by the pump 380 in the second embodiment (firstexperimental example) may have the action of removing a bubble. Further,in the second embodiment (first experimental example), the periodextending from completion of the agitation by the pump 380 until thestart of the next UV light radiation and etching voltage application(the latter four seconds of the period in which the UV light radiationand the etching voltage application are stopped (illustrated in FIG. 5))is a period for waiting for the electrolyte solution (etching liquid)320 to become still.

Next, a first variation of the eighth embodiment will be described. Asthe PEC etching proceeds, the recesses 121 and the like formed in theetching areas 111 and the like become deeper. As the recesses 121 andthe like are deeper, the bubble 130 adhering to the vicinity of thebottom of the recesses is more difficult to remove. Thus, in thisvariation, the bubble removal process is made longer as the recesses 121and the like formed are deeper.

FIG. 30 is a timing chart illustrating an example of a manner in whichPEC etching according to this variation is carried out. A differencefrom the timing chart of the eighth embodiment illustrated in FIG. 29will be described. In this variation, in the plurality of repetitions ofthe process constituted by a set including the irradiation process, thebubble removal process plus the replenishment process, and the stillnessawaiting process, the period T2 for removing the bubble 130 from thewafer 100 is made longer for a bubble removal process that is performedlater. As a result, removal of the bubble 130 when the recesses 121 andthe like are deeper can be performed more reliably.

Next, a second variation of the eighth embodiment will be described. Asthe PEC etching proceeds, the recesses 121 and the like formed in theetching areas 111 and the like become deeper. As the recesses 121 andthe like are deeper, the etching liquid 420 in the vicinity of thebottom of the recesses becomes still more easily. Thus, in thisvariation, the stillness awaiting process is made shorter as therecesses 121 and the like formed are deeper.

FIG. 31 is a timing chart illustrating an example of a manner in whichPEC etching according to this variation is carried out. A differencefrom the timing chart of the eighth embodiment illustrated in FIG. 29will be described. In this variation, in the plurality of repetitions ofthe process constituted by a set including the irradiation process, thebubble removal process plus the replenishment process, and the stillnessawaiting process, the period T3 for waiting for the etching liquid 420to become still is made shorter for a stillness awaiting process that isperformed later. As a result, the time required to complete theformation of the recesses 121 through 123 can be shortened.

Next, a third variation of the eighth embodiment will be described. FIG.32A is a schematic diagram illustrating an example of a manner in whichPEC etching according to this variation is carried out. The PEC etchingapparatus 400 of this variation is configured such that the PEC etchingapparatus 400 according to the eighth embodiment is further equippedwith an optical sensor 470, a temperature sensor 480, a temperatureadjuster 481, a bubble sensor 490, and a dichroic mirror 491.

The optical sensor 470 measures the irradiation intensity and/orirradiation energy of the UV light 431. Data corresponding to the resultof measurement by the optical sensor 470 is input in the control device440. The control device 440 controls the light irradiation device 430such that variation in the irradiation intensity or irradiation energyof the UV light 431 radiated onto the surface 100 s of the wafer 100 inthe irradiation process is limited on the basis of the measurement ofthe irradiation intensity or irradiation energy of the UV light 431 bythe optical sensor 470. In this way, according to this variation,variation in etching conditions can be limited.

The temperature sensor 480 measures the temperature of the etchingliquid 420. By being controlled by the control device 440, thetemperature adjuster 481 adjusts the temperature of the etching liquid420 to a prescribed temperature. Data corresponding to the result ofmeasurement by the temperature sensor 480 is input in the control device440. The control device 440 controls the temperature adjuster 481 suchthat variation in the temperature of the etching liquid 420 in theirradiation process is limited on the basis of the measurement of thetemperature of the etching liquid 420 by the temperature sensor 480. Adevice that is a combination of the temperature adjuster 481 and thecontrol device 440 may be regarded as a temperature adjuster. In thisway, according to this variation, variation in etching conditions can belimited.

The bubble sensor 490 measures the bubble 130 having adhered to thewafer 100. The bubble sensor 490 may be an imaging device such as a CCDcamera, for example, and observes the bubble 130 using visible light492. Data corresponding to the result of measurement by the bubblesensor 490 is input in the control device 440. The control device 440obtains the coverage rate of the bubbles 130 adhering to the wafer 100on the basis of the result of measurement of the bubbles 130 by thebubble sensor 490. A device that is a combination of the bubble sensor490 and the control device 440 may be regarded as a bubble sensor.

The dichroic mirror 491 illustrated in FIG. 32A transmits the UV light431 and reflects the visible light 492. The dichroic mirror 491 isplaced on an optical path of the UV light 431 radiated onto the wafer100 and guides the UV light 431 to the wafer 100. The dichroic mirror491 guides, to the bubble sensor 490, the visible light 492 that runsfrom the inside of the recesses 121 and the like formed in the etchingareas 111 and the like toward the dichroic mirror 491. As the visiblelight 492 used to observe the bubble 130, illumination light may beused, or if the light emitted from the light irradiation device 430includes visible light, this visible light may be used.

In this way, in this variation, it is possible to observe the bubble 130adhering to the inside of the recesses 121 and the like formed in theetching areas 111 and the like while radiating the UV light 431 used forthe PEC etching of the etching areas 111 and the like. FIG. 32Aillustrates a case where the dichroic mirror 491 is placed so as tocorrespond to a specific etching area (etching area 113 as arepresentative example) and radiation of the UV light 431 andobservation of the bubble 130 are carried out for the etching area 113by means of the dichroic mirror 491. To present another example, adichroic mirror 491 may be placed so as to correspond to all of theetching areas 111 through 113 and radiation of the UV light 431 andobservation of the bubble 130 may be carried out for the etching areas111 through 113 by means of this dichroic mirror 491. Observation usingthe bubble sensor 490 makes it possible to confirm how the PEC etchingis proceeding. The bubble 130 coverage rate obtained by using the bubblesensor 490 may be referenced for the purpose of determining the timingat which to start the period T2 for performing the bubble removalprocess.

Next, a fourth variation of the eighth embodiment will be described.FIG. 32B is a schematic diagram illustrating an example of a manner inwhich PEC etching according to this variation is carried out. Adifference from the third variation of the eighth embodiment will bedescribed. The dichroic mirror 491 illustrated in the third variation ofthe eighth embodiment is configured to transmit the UV light 431 andreflect the visible light 492, whereas the dichroic mirror 491illustrated in this variation is configured to reflect the UV light 431and transmit the visible light 492. Accordingly, in this variation, theUV light 431 reflected by the dichroic mirror 491 is radiated onto thewafer 100, and the visible light 492 transmitted by the dichroic mirror491 enters the bubble sensor 490.

The PEC etching apparatus 400 of this variation includes an illuminationdevice 493 that radiates the visible light 492 onto the wafer 100. FIG.32B illustrates the illumination device 493 that is provided fortransmitted illumination. In this example, a visible light transmissionwindow 412 is provided in the bottom of the container 410 and thevisible light 492 emitted from the illumination device 493 andtransmitted through the visible light transmission window 412 and thewafer 100 is radiated on the recesses 121 and the like formed in theetching areas 111 and the like. The visible light 492 radiated onto therecesses 121 and the like passes through the etching liquid 420 and thedichroic mirror 491 and enters the bubble sensor 490. The wafer 100,constituted by GaN over the entire thickness thereof, transmits thevisible light 492, so transmitted illumination can be used. Note,however, that although this example employs transmitted illumination, itis also possible to employ episcopic illumination.

Next, a fifth variation of the eighth embodiment will be described. FIG.33 is a schematic diagram illustrating an example of a manner in whichPEC etching according to this variation is carried out. The PEC etchingapparatus 400 of this variation is configured such that the PEC etchingapparatus 400 according to the eighth embodiment is further equippedwith a pressurized container 415.

The pressurized container 415 is configured to be capable ofaccommodating the container 410, and the PEC etching of this variationis carried out with the wafer 100 and the etching liquid 420 beingaccommodated in the pressurized container 415. Thus, in this variation,the irradiation process, bubble removal process, replenishment process,and stillness awaiting process are carried out in the pressurizedcontainer 415. In the example illustrated in FIG. 33, the pressurizedcontainer 415 includes a UV light transmission window 416, and the UVlight 431 emitted from the light irradiation device 430 disposed outsidethe pressurized container 415 is radiated onto the wafer 100 through theUV light transmission window 416. If necessary, it is possible to adopta configuration in which the light irradiation device 430 also isdisposed within the pressurized container 415.

In this example, the PEC etching is performed in the pressurizedcontainer 415 under a pressurized condition with a higher pressure thanatmosphere, so enlargement of the bubble 130 can be suppressed and anincrease in the bubble 130 coverage rate can be moderated. As such, itis possible to lengthen the period T1 of a single irradiation process,i.e. increase the number of times light is radiated (number of pulses)in intermittent radiation within a single irradiation process, as wellas reducing the number of times the bubble removal process is performed,so PEC etching can be carried out more time-efficiently.

Ninth Embodiment

Next, a ninth embodiment will be described. The eighth embodimentillustrates a manner in which the bubble 130 is removed by impartingvibration to the bubble 130. The ninth embodiment illustrates a mannerin which the bubble 130 is removed by generating a flow of the etchingliquid 420. The ninth embodiment also describes a manner in which a flowof the etching liquid 420 is generated so as to replenish the etchingliquid 420 as well as removing the bubble 130.

FIGS. 34A and 34B are schematic diagrams each illustrating an example ofa manner in which PEC etching according to a ninth embodiment is carriedout. The following example employs electrodeless PEC etching, but alsowhen employing electrode-based PEC etching, the same concept can be usedto remove bubbles 130 and replenish the etching liquid 420. The PECetching apparatus 400 of this embodiment is configured such that the PECetching apparatus 400 according to the seventh embodiment is furtherequipped with a pump 500, tank 501, external flow path 502, upstreamvalve 510, downstream valve 511, flow measurement bubble emission device520, and flow sensor 521. FIG. 34A schematically illustrates across-sectional structure of the container 410 as viewed from a lateralside direction, and FIG. 34B schematically illustrates an upper-sidestructure of the container 410 as viewed from the upper side. Note thatin FIG. 34B, part of the equipment composing the PEC etching apparatus400 are omitted to simplify illustration.

In the container 410 of this embodiment, openings 417 and 418 areprovided at ends of the container 410 on the upstream side anddownstream side, respectively, in terms of a flow 421 of the etchingliquid 420 that is generated. The external flow path 502 is provided soas to connect the downstream opening 418 and the upstream opening 417 ofthe container 410. The container 410 and the external flow path 502 forma flow path through which the etching liquid 420 circulates. The pump500 is disposed on an intermediate point of the external flow path 502.The etching liquid 420 circulates as a result of the pump 500 generatingthe flow 421 that traverses the interior of the container 410 from theupstream opening 417 toward the downstream opening 418.

The pump 500 generates the flow 421 of the etching liquid 420 in thecontainer 410 to remove (detach) the bubble 130 adhering to the wafer100 from the wafer 100. The pump 500 is another example of the bubbleremoval device. The pump 500 performs a prescribed operation by beingcontrolled by the control device 440. A device that is a combination ofthe pump 500 and the control device 440 may be regarded as a pump.

The upstream valve 510 switches between a state in which the etchingliquid 420 can pass through the upstream opening 417 of the container410 and a state in which same cannot pass therethrough. The downstreamvalve 511 switches between a state in which the etching liquid 420 canpass through the downstream opening 418 of the container 410 and a statein which same cannot pass therethrough. Each of the upstream valve 510and the downstream valve 511 performs a prescribed operation by beingcontrolled by the control device 440. A device that is a combination ofthe upstream valve 510 and the control device 440 may be regarded as anupstream valve, and a device that is a combination of the downstreamvalve 511 and the control device 440 may be regarded as a downstreamvalve.

The tank 501 is disposed on an intermediate point of the external flowpath 502. A new etching liquid 420 accommodated in the tank 501 issupplied into the container 410 while being merged with the flow 421 bythe pump 500. In this way, the etching liquid 420 is replenished in thecontainer 410. The old etching liquid 420 discharged from the container410 as the new etching liquid 420 is supplied may be discarded(recovered). However, as long as there is no significant impact on theetching result (if the concentrations of the various components are notchanged significantly), the etching liquid 420 in the container 410 maybe increased in accordance with the supply of the new etching liquid420. The pump 500 and the tank 501 are another example of the etchingliquid replenishment device.

The flow (movement) measurement bubble emission device 520 emits abubble 522 formed from N₂ gas, or the like, into the etching liquid 420.The flow measurement bubble emission device 520 may be disposed, forexample, downstream from the wafer 100 in the flow 421. The flow(movement) sensor 521 may be an imaging device such as a CCD camera, forexample, and measures (observes) the bubble 521 in the etching liquid420. Data corresponding to the result of measurement by the flow sensor521 is input in the control device 440. The control device 440 uses theresult of measurement by the flow sensor 521 to determine whether theetching liquid 420 is flowing (moving), i.e. whether the etching liquid420 is still. A device that is a combination of the flow sensor 521 andthe control device 440 may be regarded as a flow (movement) sensor.

FIG. 35 is a timing chart illustrating an example of a manner in whichPEC etching according to the ninth embodiment is carried out, andillustrates a manner in which the UV light 431 may be radiated onto thewafer 100 and a manner in which the pump 500, the upstream valve 510,and the downstream valve 511 may operate. The irradiation processperformed in the period T1 is equivalent to the eighth embodiment.

After the irradiation process, the pump 500 is operated in the periodT2. At the timing when the period T2 is started, the upstream valve 510and the downstream valve 511 are opened. Thus, the flow 421 of theetching liquid 420 is generated in the container 410 so as to carry outthe bubble removal process and the replenishment process.

At the timing when the period T2 is ended, the upstream valve 510 andthe downstream valve 511 are closed. Closing the upstream valve 510 andthe downstream valve 511 at the timing when the period T2 is ended maybe regarded as a process of attenuating the flow of the etching liquid420. The upstream valve 510 and the downstream valve 511 are an exampleof a flow attenuation device that attenuates the flow of the etchingliquid 420.

After the bubble removal process and the replenishment process, in theperiod T3, the stillness awaiting process is carried out. In thestillness awaiting process, whether the etching liquid 420 has becomestill is determined on the basis of the measurement by the flow sensor521 (sensor for measuring movement of the etching liquid 420). In otherwords, the length of the period T3 is set as the time required todetermine that the etching liquid 420 has become still on the basis ofthe measurement by the flow sensor 521.

Similarly to the eighth embodiment, a process constituted by a setincluding the irradiation process, the bubble removal process plus thereplenishment process, and the stillness awaiting process is repeated aplurality of times. When the recesses 121 and the like reach aprescribed depth (i.e. when the formation of the recesses 121 and thelike is complete), the PEC etching is terminated.

In this embodiment as well, similarly to the eighth embodiment, throughremoval of bubbles 130, it is possible to limit the occurrence of a casewhere proper light radiation is difficult due to adhesion of the bubble130 on the wafer 100. Moreover, through replenishment of the etchingliquid 420, it is possible to limit the occurrence of a case where thePEC etching cannot proceed properly due to degradation of the etchingliquid 420.

Note that in the irradiation process (period T1), the irradiationprocess may be carried out while performing the measurement using theflow sensor 521 to confirm that the etching liquid 420 is still. For thepurpose of attenuating more reliably the flow 421 of the etching liquid420 generated in the bubble removal process and replenishment process, ashutter device that blocks the flow 421 may be added.

Other Embodiments

Embodiments and variations of the present invention have been describedabove by way of specific examples. However, the present invention is notlimited to the above embodiments and variations, and can undergo, forexample, changes, improvements, or combinations in various ways withinthe scope of the invention.

The aforementioned PEC etching can be used preferably as part of amethod for producing a semiconductor device in which a GaN material isused. For example, this PEC etching can be used for a structureformation method when producing a Schottky barrier diode using the GaNmaterial 100 according to the second embodiment (an epitaxial substrate30 in which an epitaxial layer 20 is constituted by a GaN layer to whichn-type impurities have been added).

To cite another example, this PEC etching can be used for a structureformation method when producing a superjunction (SJ) structure byforming a trench in an n-type area of an epitaxial layer 20 andregrowing (loading) a p-type epitaxial layer in the trench (see FIG.22A); here, the conductive type may be reversed and an n-type epitaxiallayer may be regrown (loaded) in a trench formed in a p-type epitaxiallayer 20.

Furthermore, the aforementioned PEC etching can be used as a structureformation method when producing a pn junction diode or a transistorusing the GaN material 100 according to the third embodiment (anepitaxial substrate 30 in which an epitaxial layer 20 includes a GaNlayer to which n-type impurities have been added and a GaN layer towhich p-type impurities have been added). For example, this PEC etchingcan be used for the formation of a mesa structure or the formation of aridge structure of a laser diode.

It is also possible to carry out processing in which this PEC etching isused to remove only a p-type GaN layer constituting the surface layer ofan n-p layered structure, as exemplified by the case of producing ajunction barrier Schottky (JBS) diode.

The constitution of the epitaxial layer 20 can be selected, asappropriate, according to need and, for example, may include a GaN layerto which no electroconductive impurities are added or may be constitutedby a layered structure of, for example, n-p-n. This PEC etching may becarried out exclusively on a specific layer of an epitaxial layer 20having a layered structure. The GaN substrate is not limited to thesubstrate 10 described in the first embodiment and a GaN substratehaving an area having an adequately low dislocation density (forexample, lower than 1×10⁷/cm²) is used preferably. Theelectroconductivity of the substrate 10 may be selected, as appropriate.

To give an example, a metal-insulator-semiconductor field effecttransistor (MISFET) of a trench gate structure may be produced in thefollowing way. A layered structure of n-p-n (or p-n-p) is adopted forthe epitaxial layer 20; a recess 40 is formed in the epitaxial layer 20using PEC etching; and a npn junction (or pnp junction) serving as theoperation part for the transistor is formed on the side face 23 of therecess 40. An insulated gate electrode is formed in the recess 40 (seeFIG. 22A), in addition to which a source electrode and a drain electrodeto be electrically connected to the n layers of the npn layeredstructure (or the p layers of the pnp layered structure) are formed.With this production method, the PEC etching can be used to form a MISinterface, at which the npn junction (or pnp junction) serving as theoperation part for a semiconductor device is located, while incurringlittle damage and the resultant interface having superior flatness;thus, a semiconductor device having high operation performance can beproduced simply.

The electrode structure when producing a semiconductor device using theGaN material 100 may differ according to the electroconductivecharacteristics of the substrate 10. For example, the structure of anelectrode to be electrically connected to an n-type GaN layer formed onthe (front) surface of the substrate 10 may be as follows. For example,when producing a light-emitting diode (LED) using an n-type conductivesubstrate 10, the electrode may be formed on the rear surface of thesubstrate 10. Meanwhile, when, for example, producing a GaN-highelectron mobility transistor (HEMT) using a semi-insulating substrate10, the electrode will be formed on the n-type GaN layer, i.e. on thesurface side of the substrate 10.

Application of the aforementioned PEC etching is not limited tosemiconductor devices such as diodes or transistors, and more generally,this PEC etching may be used preferably as a method for producingstructures in which GaN material is used. In addition to theaforementioned manners of usage, the PEC etching may also be used, forexample, when dicing wafer that is formed from a GaN material or for theformation of a component of micro-electro-mechanical systems (MEMS) inwhich GaN is used. This PEC etching may also be used to etch theentirety of the principal face of a GaN material.

As in the case where a recess having a bottom face or a protrusion isformed, both the bottom face and the side face formed using PEC etchingmay sometimes be exposed on the surface of the GaN material having beenetched. Alternatively, as in the case where a GaN material is penetratedthrough (e.g. when a through-hole is formed or the GaN material isdivided into segments), only the side face formed using PEC etching maybe exposed on the surface of the GaN material having been etched.Further, as in the case where an entire face is etched, only the bottomface formed using PEC etching may be exposed on the surface of the GaNmaterial having been etched.

When etching is carried out to penetrate through the GaN material, suchas when a through-hole is formed or the GaN material is divided, the GaNmaterial having been etched has an area in which the side face formedusing the PEC etching is exposed throughout the entire thickness. Thisside face has a property and shape that are similar to what has beendescribed in the fourth and fifth embodiments as a trace of PEC etching.When the entire face is etched, the GaN material having been etched hasan area in which the bottom face formed using PEC etching is exposedentirely. This bottom face has a property and shape that are similar towhat has been described in the fourth and fifth embodiments as a traceof PEC etching.

FIG. 21 is a schematic diagram illustrating an example of a situationwhere PEC etching is carried out to penetrate through a GaN material100, such as when a through-hole is formed or the GaN material isdivided. In a case like this, a seal member 42 may be provided on thebottom face and the side face of the GaN material 100 according to need,so that the electrolyte solution does not leak in the course ofpenetration.

The aforementioned embodiments illustrate examples of a case where PECetching is carried out in the depth direction from the c face which hasa large area and with which it is easy to form a plurality ofstructures, but illustrating such examples does not deprive the presentinvention of applications in which PEC etching is carried out in thedepth direction from other crystal faces.

The structure produced using the PEC etching described above may beconstituted by a combination of the GaN member having a structure formedusing the PEC etching and another member. Note that the state in which amask used in PEC etching is formed (remains) on a GaN member that has astructure formed using the PEC etching (see, for example, FIG. 18A) maybe regarded as an intermediate structure used to obtain a finalstructure. When a certain member is loaded inside a recess formed usingthe aforementioned PEC etching and thus the recess is not exposed to theouter face of the structure, the structure includes the recessregardless. Likewise, even when a certain member is loaded on theoutside of a protrusion formed using the aforementioned PEC etching andthus the protrusion is not exposed to the outer face of the structure,the structure includes the protrusion regardless. FIG. 22A is aschematic diagram illustrating an example of a situation where a fillingmember 50 is loaded into a recess 40 formed using PEC etching. FIG. 22Bis a schematic diagram illustrating an example of a situation where afilling member 50 is loaded on the outside of a protrusion 45 formedusing PEC etching.

The first experimental example described in the second embodimentindicates a finding that from the perspective of limiting excessivebubble generation, it is preferable to limit the (saturation) currentdensity in the electrode-based PEC etching to no more than 3 mA/cm².

The seventh through ninth embodiments and the variations thereofdescribe electrodeless PEC etching. With the electrodeless PEC etching,no cathode electrode is required, so it is not necessary that the wafer100 subject to etching be conductive over the entire thickness thereof.Accordingly, the wafer 100 may not be formed from GaN over the entirethickness thereof and it is sufficient if at least the surface (etchingsurface) of the wafer 100 is formed from GaN (more typically, a groupIII nitride). As such, the electrodeless PEC etching described in theseembodiments and variations may be applied to etching of a GaN layer(more typically, a group III nitride layer) grown on a sapphiresubstrate, silicon carbide (SiC) substrate, silicon (Si) substrate, orother such substrates of a different species.

When forming a penetrating structure in the wafer 100 (formed from amaterial subject to PEC etching over the entire thickness thereof), itis more preferable to use the electrodeless PEC etching that doesrequire a sealing structure.

The effect of enhancing etching uniformity in the PEC etching performedon the wafer 100 having a large diameter of, for example, two inches ormore, as described in the seventh embodiment and the variations thereof,can be achieved not only for PEC etching of GaN but also PEC etching ofother group III nitrides than GaN. This effect of enhancing etchinguniformity can be achieved without being particularly limited by thedislocation density of the group III nitride layer subject to etching(even when the dislocation density is 1×10⁷/cm² or higher, for example).In other words, the effect of enhancing etching uniformity can beachieved even when performing PEC etching on, for example, a group IIInitride layer that is epitaxially grown on a substrate of a differentspecies, e.g. sapphire substrate, and has a dislocation density of,e.g., 1×10⁸/cm² or more.

The effect of limiting variation in etching conditions through removalof the bubble 130 or replenishment of the etching liquid 420, asdescribed in the eighth and ninth embodiments and the variationsthereof, can be achieved not only for PEC etching of GaN but also PECetching of other group III nitrides than GaN.

Thus, the seventh through ninth embodiments and the variations thereofcan be applied preferably in carrying out PEC etching on a wafer 100 atleast having the surface thereof formed from a group III nitridecrystal.

The group III element included in the group III nitride may be at leastone from among aluminum (Al), Ga, and indium (In). UV light with awavelength of shorter than 365 nm, for example, may be used for the UVlight 431 radiated onto the wafer 100. Using PEC etching of GaN as areference, when Al is included, UV light with a shorter wavelength maybe used, and when In is included, light with a longer wavelength can beused. In other words, according to the composition of the group IIInitride to be processed, appropriate light with a wavelength allowingthe group III nitride to be PEC etched can be selected for use.

The concept of PEC etching in relation to the Al component or Incomponent in the group III nitride is equivalent to the conceptdescribed with reference to (chem. 1) and (chem. 2) concerning the Gacomponent. That is, the PEC etching may be carried out by generating anoxide of Al or an oxide of In by UV light radiation and dissolving theoxide. As described above, the group III nitride subject to this PECetching is not limited to GaN.

The seventh through ninth embodiments and the variations thereofillustrate manners in which a mask 41 having an opening for exposing theetching areas 111 and the like is formed on the wafer 100. By shaping(patterning) the irradiation cross-section of the UV light 431 into anirradiation cross-section with which only the etching areas 111 and thelike are irradiated, theoretically it is possible to carry out masklessPEC etching. A DMD may be used for shaping the irradiationcross-section, for example. According to the requirements, the mask 41used may be constituted by a mask that does not block light or anon-metal (non-conductive) mask.

The above description illustrates PEC etching in which an oxide of agroup III element such as Ga₂O₃ is dissolved using an alkaline solutionand presents a NaOH solution as an example of the alkaline solution, butthe alkaline solution used in the PEC etching may also be a potassiumhydroxide (KOH) solution, for example. Note that the PEC etching mayalso be performed by using an acidic solution to dissolve an oxide of agroup III element. A phosphoric acid (H₃PO₄) solution is an example ofthe acidic solution that may be used for the PEC etching.

The etching liquid used for the electrodeless PEC etching may be analkaline or acidic etching liquid that includes oxygen used forgenerating an oxide of the group III element included in the group IIInitride to be etched and also includes an oxidizing agent that acceptselectrons. Peroxodisulfate ion (S₂O₈ ²) is an example of such anoxidizing agent. Note that although the above example illustrates amanner in which S₂O₈ ²⁻ is supplied from potassium peroxodisulfate(K₂S₂O₈), S₂O₈ ²⁻ may also be supplied from other sources includingsodium peroxodisulfate (Na₂S₂O₈) and ammonium peroxodisulfate (ammoniumpersulfate, (NH₄)₂S₂O₈).

<Preferable Aspects of the Present Invention>

Preferable aspects of the present invention will be supplementarilydescribed hereafter.

(Supplementary Description 1)

A structure production method including

a process of preparing a wafer having a diameter of two inches or more,at least a surface of the wafer being formed from a group III nitridecrystal,

a process of preparing an etching liquid in a container,

a process of accommodating the wafer in the container in a condition inwhich the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with a surface ofthe etching liquid, and

a process of radiating light (with a wavelength of shorter than 365 nm)onto the surface of the wafer in a condition in which the wafer and theetching liquid are still,

wherein

a first etching area and a second etching area are defined on thesurface of the wafer, the first and second etching areas being disposedat an interval from each other, the first and second etching areas beingareas where the group III nitride crystal is to be etched due to thesurface of the wafer being irradiated with the light in a condition inwhich the surface is immersed in the etching liquid, and

in the process of radiating the light onto the surface of the wafer, thelight is radiated perpendicularly onto each of a surface of the firstetching area and a surface of the second etching area.

(Supplementary Description 2)

The structure production method according to Supplementary description1, wherein in the process of radiating the light onto the surface of thewafer, parallel light forming the light is radiated onto the firstetching area and the second etching area.

(Supplementary Description 3)

The structure production method according to Supplementary description 1or 2, wherein in the process of radiating the light onto the surface ofthe wafer, parallel light forming the light and having an irradiationcross-section of a size encompassing the first and second etching areason the surface of the wafer is radiated.

(Supplementary Description 4)

The structure production method according to any one of Supplementarydescriptions 1 through 3, wherein in the process of radiating the lightonto the surface of the wafer, the light having an identical irradiationintensity for the first etching area and the second etching area isradiated.

(Supplementary Description 5)

The structure production method according to any one of Supplementarydescriptions 1 through 4, wherein in the process of radiating the lightonto the surface of the wafer, the light is radiated in a manner suchthat cumulative irradiation energy in the first etching area andcumulative irradiation energy in the second etching area are identical.

(Supplementary Description 6)

The structure production method according to any one of Supplementarydescriptions 1 through 5, wherein in the process of radiating the lightonto the surface of the wafer, the light is radiated onto the firstetching area and the second etching area simultaneously.

(Supplementary description 7)

The structure production method according to Supplementary description6, wherein in the process of radiating the light onto the surface of thewafer, the light having an irradiation cross-section of a sizeencompassing the first and second etching areas on the surface of thewafer is radiated, an irradiation intensity distribution in theirradiation cross-section being uniform.

(Supplementary Description 8)

The structure production method according to any one of Supplementarydescriptions 1 through 5, wherein in the process of radiating the lightonto the surface of the wafer, the light is radiated onto the firstetching area and the second etching area asynchronously.

(Supplementary Description 9)

The structure production method according to Supplementary description8, wherein in the process of radiating the light onto the surface of thewafer, an irradiation cross-section of the light is moved over thesurface of the wafer.

(Supplementary Description 10)

The structure production method according to any one of Supplementarydescriptions 1 through 9, wherein in the process of radiating the lightonto the surface of the wafer, the light is radiated intermittently ontoeach of the first etching area and the second etching area.

(Supplementary Description 11)

The structure production method according to any one of Supplementarydescriptions 1 through 10, wherein in the process of radiating the lightonto the surface of the wafer, each of the first etching area and thesecond etching area is etched preferably to a depth of 8 μm or more,more preferably to a depth of 10 μm or more.

(Supplementary Description 12)

The structure production method according to any one of Supplementarydescriptions 1 through 11, wherein a distance from the surface of thewafer to the surface of the etching liquid is preferably between 1 mmand 100 mm (inclusive), more preferably between 3 mm and 100 mm(inclusive), even more preferably between 5 mm and 100 mm (inclusive).

(Supplementary Description 13)

The structure production method according to any one of Supplementarydescriptions 1 through 12, the method including

a process of preparing another wafer having a diameter of two inches ormore, at least a surface of the other wafer being formed from the groupIII nitride crystal,

a process of accommodating the other wafer in the container in acondition in which a surface of the other wafer is immersed in theetching liquid in a manner such that the surface of the other wafer isparallel with the surface of the etching liquid and the distance fromthe surface of the wafer to the surface of the etching liquid and adistance from the surface of the other wafer to the surface of theetching liquid are identical, and

a process of radiating the light onto the surface of the other wafer ina condition in which the other wafer and the etching liquid are still,

wherein

a third etching area and a fourth etching area are defined on thesurface of the other wafer, the third and fourth etching areas beingdisposed at an interval from each other, the third and fourth etchingareas being areas where the group III nitride crystal is to be etcheddue to the surface of the other wafer being irradiated with the light ina condition in which the surface is immersed in the etching liquid, and

in the process of radiating the light onto the surface of the otherwafer, the light is radiated perpendicularly onto each of a surface ofthe third etching area and a surface of the fourth etching area.

(Supplementary Description 14)

The structure production method according to Supplementary description13, wherein in the process of radiating the light onto the surface ofthe wafer and in the process of radiating the light onto the surface ofthe other wafer, the light is radiated in a manner such that irradiationintensity or cumulative irradiation energy is identical in the first,second, third, and fourth etching areas.

(Supplementary Description 15)

The structure production method according to Supplementary description13 or 14, wherein in the process of radiating the light onto the surfaceof the wafer and in the process of radiating the light onto the surfaceof the other wafer, the light having an irradiation cross-section of asize encompassing the first, second, third, and fourth etching areas isradiated, an irradiation intensity distribution in the irradiationcross-section being uniform.

(Supplementary Description 16)

The structure production method according to any one of Supplementarydescriptions 1 through 15, wherein the method does not employ a cathodeelectrode immersed in the etching liquid while being connected to a wireextending to outside the etching liquid (does not employ a cathodeelectrode immersed in the etching liquid and a wire connected to thecathode electrode and extending to outside the etching liquid), and

the etching liquid is an alkaline or acidic etching liquid containing anoxidizing agent that accepts an electron.

(The structure production method according to any one of Supplementarydescriptions 1 through 15, wherein the etching liquid contains ahydroxide ion and a peroxodisulfate ion, and

the group III nitride crystal forming the surface of the wafer is etchedwithout using a cathode electrode immersed in the etching liquid.)

(Supplementary Description 17)

The structure production method according to any one of Supplementarydescriptions 1 through 15, wherein

the etching liquid contains a hydroxide ion, and

the group III nitride crystal forming the surface of the wafer is etchedusing a cathode electrode immersed in the etching liquid.

(Supplementary Description 18)

The structure production method according to Supplementary description16, wherein the group III nitride crystal forming the surface of thewafer is etched in a condition in which an etching voltage is appliedbetween the cathode electrode and the group III nitride crystal.

(Supplementary Description 19)

The structure production method according to Supplementary description18, wherein the group III nitride crystal forming the surface of thewafer includes GaN including an area in which dislocation density isless than 1×10⁷/cm², and

the etching voltage is preferably a voltage falling within a range of0.16 V to 1.30 V (inclusive), more preferably a voltage falling within arange of 0.52 V to 1.15 V (inclusive).

(Supplementary Description 20)

The structure production method according to any one of Supplementarydescriptions 1 through 19, wherein variation in irradiation intensity orirradiation energy of the light radiated onto the surface of the waferin the process of radiating the light onto the surface of the wafer islimited on the basis of measurement of the irradiation intensity orirradiation energy of the light by an optical sensor.

(Supplementary Description 21)

The structure production method according to any one of Supplementarydescriptions 1 through 20, wherein variation in temperature of theetching liquid in the process of radiating the light onto the surface ofthe wafer is limited on the basis of measurement of the temperature ofthe etching liquid by a temperature sensor.

(Supplementary Description 22)

The structure production method according to any one of Supplementarydescriptions 1 through 21, wherein the method includes a process ofremoving (detaching) a bubble, which is generated due to the etching ofthe group III nitride crystal and which adheres to the wafer, from thewafer.

(Supplementary Description 23)

The structure production method according to Supplementary description22, wherein in the process of removing the bubble from the wafer, thebubble is removed by imparting vibration to the bubble.

(Supplementary Description 24)

The structure production method according to Supplementary description22 or 23, wherein in the process of removing the bubble from the wafer,the bubble is removed by generating a flow of the etching liquid.

(Supplementary Description 25)

A structure production apparatus including

a container configured to accommodate a wafer and an etching liquid, thewafer having a diameter of two inches or more, at least a surface of thewafer being formed from a group III nitride crystal, and

a light irradiation device configured to radiate light (with awavelength of shorter than 365 nm) onto the surface of the wafer,wherein

the container accommodates the wafer inside the container in a conditionin which the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with a surface ofthe etching liquid, and retains the wafer and the etching liquid in acondition in which the wafer and the etching liquid are still, and

the light irradiation device radiates the light perpendicularly onto asurface of each of a first etching area and a second etching area, thefirst and second etching areas being defined on the surface of thewafer, the first and second etching areas being disposed at an intervalfrom each other, the first and second etching areas being areas wherethe group III nitride crystal is to be etched due to the surface of thewafer being irradiated with the light in a condition in which thesurface is immersed in the etching liquid.

(Supplementary Description 26)

The structure production apparatus according to Supplementarydescription 25, wherein the light irradiation device radiates the lightin the form of parallel light onto the first etching area and the secondetching area.

(Supplementary Description 27)

The structure production apparatus according to Supplementarydescription 25 or 26, wherein the light irradiation device radiatesparallel light having an irradiation cross-section of a sizeencompassing the first and second etching areas on the surface of thewafer.

(Supplementary Description 28)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 27, wherein the light irradiation deviceradiates the light having an identical irradiation intensity for thefirst etching area and the second etching area.

(Supplementary Description 29)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 28, wherein the light irradiation deviceradiates the light in a manner such that cumulative irradiation energyin the first etching area and cumulative irradiation energy in thesecond etching area are identical.

(Supplementary Description 30)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 29, wherein the light irradiation deviceradiates the light onto the first etching area and the second etchingarea simultaneously.

(Supplementary Description 31)

The structure production apparatus according to Supplementarydescription 30, wherein the light irradiation device radiates the lighthaving an irradiation cross-section of a size encompassing the first andsecond etching areas on the surface of the wafer, an irradiationintensity distribution in the irradiation cross-section being uniform.

(Supplementary Description 32)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 29, wherein the light irradiation deviceradiates the light onto the first etching area and the second etchingarea asynchronously.

(Supplementary Description 33)

The structure production method according to Supplementary description32, wherein the light irradiation device moves the irradiationcross-section of the light over the surface of the wafer.

(Supplementary Description 34)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 33, wherein the light irradiation deviceradiates the light intermittently onto each of the first etching areaand the second etching area.

(Supplementary Description 35)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 34, wherein the light irradiation deviceincludes at least one from among an ultraviolet light emitting diode, anultraviolet laser, and an ultraviolet lamp as a light source that emitsthe light.

(Supplementary Description 36)

The structure production method according to any one of Supplementarydescriptions 25 through 35, wherein the container accommodates the waferand the etching liquid in a condition in which a distance from thesurface of the wafer to the surface of the etching liquid is preferablybetween 1 mm and 100 mm (inclusive), more preferably between 3 mm and100 mm (inclusive), even more preferably between 5 mm and 100 mm(inclusive).

(Supplementary Description 37)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 36, wherein

the container accommodates, together with the wafer and the etchingliquid, another wafer having a diameter of two inches or more, at leasta surface of the other wafer being formed from the group III nitridecrystal,

the container accommodates the other wafer inside the container andretains the other wafer and the etching liquid in a condition in whichthe other wafer and the etching liquid are still and the surface of theother wafer is immersed in the etching liquid in a manner such that thesurface of the other wafer is parallel with the surface of the etchingliquid and the distance from the surface of the wafer to the surface ofthe etching liquid and the distance from the surface of the other waferto the surface of the etching liquid are identical, and

the light irradiation device radiates the light perpendicularly onto asurface of each of a third etching area and a fourth etching area, thethird and fourth etching areas being defined on the surface of the otherwafer, the third and fourth etching areas being disposed at an intervalfrom each other, the third and fourth etching areas being areas wherethe group III nitride crystal is to be etched due to the surface of theother wafer being irradiated with the light in a condition in which thesurface is immersed in the etching liquid.

(Supplementary Description 38)

The structure production apparatus according to Supplementarydescription 37, wherein the light irradiation device radiates the lightin a manner such that irradiation intensity or cumulative irradiationenergy is identical in the first, second, third, and fourth etchingareas.

(Supplementary Description 39)

The structure production apparatus according to Supplementarydescription 37 or 38, wherein the light irradiation device radiates thelight having an irradiation cross-section of a size encompassing thefirst, second, third, and fourth etching areas, an irradiation intensitydistribution in the irradiation cross-section being uniform.

(Supplementary Description 40)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 39, wherein the apparatus does not include acathode electrode immersed in the etching liquid while being connectedto a wire extending to outside the etching liquid (does not include acathode electrode immersed in the etching liquid and a wire connected tothe cathode electrode and extending to outside the etching liquid).(Preferably, the etching liquid used is an alkaline or acidic etchingliquid containing an oxidizing agent that accepts an electron.)

(Supplementary Description 41)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 40, wherein

the apparatus includes

a cathode electrode immersed in the etching liquid,

a wire configured to electrically connect the cathode electrode and thegroup III nitride crystal forming the surface of the wafer, and

a voltage source configured to apply an etching voltage between thecathode electrode and the group III nitride crystal forming the surfaceof the wafer, and

the voltage source applies, as the etching voltage, preferably a voltagefalling within a range of 0.16 V to 1.30 V (inclusive), more preferablya voltage falling within a range of 0.52 V to 1.15 V (inclusive).

(Supplementary Description 42)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 41, wherein

the apparatus includes an optical sensor, and

the light irradiation device limits variation in irradiation intensityor irradiation energy of the light radiated onto the surface of thewafer on the basis of measurement of the irradiation intensity orirradiation energy of the light by the optical sensor.

(Supplementary Description 43)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 42, wherein

the apparatus includes a temperature sensor and a temperature adjuster,and

the temperature adjuster limits variation in temperature of the etchingliquid on the basis of measurement of the temperature of the etchingliquid by the temperature sensor.

(Supplementary Description 44)

The structure production apparatus according to any one of Supplementarydescriptions 25 through 43, wherein the apparatus includes a bubbleremoval device configured to remove (detach) a bubble, which isgenerated due to the etching of the group III nitride crystal and whichadheres to the wafer, from the wafer.

(Supplementary Description 45)

The structure production apparatus according to Supplementarydescription 44, wherein the bubble removal device removes the bubble byimparting vibration to the bubble.

(Supplementary Description 46)

The structure production method according to Supplementary description44 or 45, wherein the bubble removal device removes the bubble bygenerating a flow of the etching liquid.

(Supplementary Description 47)

A structure production method including

a process of preparing a wafer having a diameter of two inches or more,at least a surface of the wafer being formed from a group III nitridecrystal,

a process of preparing an alkaline or acidic etching liquid in acontainer, the etching liquid containing a peroxodisulfate ion as anoxidizing agent that accepts an electron,

a process of accommodating the wafer in the container in a condition inwhich the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with a surface ofthe etching liquid, and

a process of radiating light from the surface side of the etching liquidonto the surface of the wafer in a condition in which the wafer and theetching liquid are still, i.e. without agitating the etching liquid,

wherein

a first etching area and a second etching area are defined on thesurface of the wafer, the first and second etching areas being disposedat an interval from each other, the first and second etching areas beingareas where the group III nitride crystal is to be etched due to thesurface of the wafer being irradiated with the light in a condition inwhich the surface is immersed in the etching liquid, and

in the process of radiating the light onto the surface of the wafer, thelight is radiated perpendicularly onto each of a surface of the firstetching area and a surface of the second etching area. The structureproduction method may encompass any of the configurations set forth inSupplementary descriptions 2 through 16, for example.

(Supplementary Description 48)

The structure production method according to Supplementary description47, wherein a sulfate ion radical generated as a result of the lightbeing radiated on the peroxodisulfate ion is supplied to the first andsecond etching areas by diffusion.

(Supplementary Description 49)

The structure production method according to Supplementary description47 or 48, wherein

in the process of radiating the light onto the surface of the wafer, thelight is radiated intermittently onto each of the first etching area andthe second etching area, and

within the intermittent radiation of the light,

-   -   generating, in a radiation period in which the light is        radiated, an oxide of a group III element included in the group        III nitride crystal, and    -   dissolving the entire thickness of the oxide in a non-radiation        period in which the light is not radiated are repeated        alternately.        (Supplementary Description 50)

The structure production method according to Supplementary description49, wherein in the non-radiation period, an electron accumulated on thewafer in the radiation period is consumed by nonradiative recombination.

(Supplementary Description 51)

A structure production apparatus including

a container configured to accommodate a wafer and an alkaline or acidicetching liquid, the wafer having a diameter of two inches or more, atleast a surface of the wafer being formed from a group III nitridecrystal, the etching liquid containing a peroxodisulfate ion as anoxidizing agent that accepts an electron, and

a light irradiation device configured to radiate light from a surfaceside of the etching liquid onto the surface of the wafer,

wherein

the container accommodates the wafer inside the container in a conditionin which the surface of the wafer is immersed in the etching liquid in amanner such that the surface of the wafer is parallel with the surfaceof the etching liquid, and retains the etching liquid in a manner suchthat the light is radiated onto the surface of the wafer in a conditionin which the wafer and the etching liquid are still, i.e. in a conditionin which the etching liquid is not agitated, and

the light irradiation device radiates the light perpendicularly onto asurface of each of a first etching area and a second etching area, thefirst and second etching areas being defined on the surface of thewafer, the first and second etching areas being disposed at an intervalfrom each other, the first and second etching areas being areas wherethe group III nitride crystal is to be etched due to the surface of thewafer being irradiated with the light in a condition in which thesurface is immersed in the etching liquid. The structure productionapparatus may encompass any of the configurations set forth inSupplementary descriptions 26 through 40, for example.

(Supplementary Description 52)

The structure production apparatus according to Supplementarydescription 51, wherein a sulfate ion radical generated as a result ofthe light being radiated on the peroxodisulfate ion is supplied to thefirst and second etching areas by diffusion.

(Supplementary Description 53)

The structure production apparatus according to Supplementarydescription 51 or 52, wherein the light irradiation device radiates thelight intermittently onto each of the first etching area and the secondetching area, and

within the intermittent radiation of the light,

-   -   generating, in a radiation period in which the light is        radiated, an oxide of a group III element included in the group        III nitride crystal, and    -   dissolving the entire thickness of the oxide in a non-radiation        period in which the light is not radiated

are repeated alternately.

(Supplementary Description 54)

The structure production apparatus according to Supplementarydescription 53, wherein in the non-radiation period, an electronaccumulated on the wafer in the radiation period is consumed bynonradiative recombination.

DESCRIPTION OF REFERENCE SYMBOLS

-   1: underlying substrate-   2: underlying layer-   2 a: void-including layer-   3: metal layer-   3 a: nanomask-   4: void-formed substrate-   5: void-   6: crystal-   10: GaN substrate-   20: GaN layer (epitaxial layer)-   21 n: GaN layer to which n-type impurities have been added-   21 p: GaN layer to which p-type impurities have been added-   22: area subject to etching-   23: side face-   23 pn: pn junction-   30: epitaxial substrate-   40: recess-   41: mask-   42: seal member-   45: protrusion-   50: filling member-   100: GaN material (wafer)-   100 s: surface (etching face) of wafer-   111, 112, 113: etching area-   121, 122, 123: recess-   130: bubble-   6 s, 10 s, 20 s: principal face-   200: HVPE device-   300: electrochemical cell-   310: container-   320: electrolyte solution-   330: cathode electrode-   340: anode electrode-   350: wire-   360: voltage source-   370: light source-   371: UV light-   380: pump-   400: PEC etching apparatus (structure production apparatus)-   410: container-   411: support-   412: visible light transmission window-   416: UV light transmission window-   417: upstream opening-   418: downstream opening-   420: etching liquid-   420 s: surface of etching liquid-   421: flow-   430: light irradiation device-   431: UV light-   440: control device-   450: vibration generator-   460: pump-   461: tank-   470: optical sensor-   480: temperature sensor-   481: temperature adjuster-   490: bubble sensor-   491: dichroic mirror-   492: visible light-   493: illumination device-   500: pump-   501: tank-   502: external flow path-   510: upstream valve-   511: downstream valve-   520: flow measurement bubble emission device-   521: flow sensor-   522: bubble

The invention claimed is:
 1. A structure production method comprisingpreparing a wafer having a diameter of two inches or more, at least asurface of the wafer being formed from a group III nitride crystal,preparing an alkaline or acidic etching liquid in a container, theetching liquid containing a peroxodisulfate ion as an oxidizing agentthat accepts an electron, accommodating the wafer in the container in acondition in which the surface of the wafer is immersed in the etchingliquid in a manner such that the surface of the wafer is parallel with asurface of the etching liquid, and radiating light onto the surface ofthe wafer from the surface side of the etching liquid heated so that asulfate ion radical is generated, wherein the surface of the waferincludes a first etching area and a second etching area, the first andsecond etching areas being disposed at an interval from each other, thefirst and second etching areas being areas where the group III nitridecrystal is to be etched due to the surface of the wafer being irradiatedwith the light in a condition in which the surface is immersed in theetching liquid, and a non-conductive mask is formed on the surface ofthe wafer, and in radiating the light onto the surface of the wafer, thelight is radiated perpendicularly onto each of a surface of the firstetching area and a surface of the second etching area.
 2. The structureproduction method according to claim 1, wherein in radiating the lightonto the surface of the wafer, the light is radiated onto the firstetching area and the second etching area simultaneously.
 3. Thestructure production method according claim 1, wherein in radiating thelight onto the surface of the wafer, the light is radiated onto thefirst etching area and the second etching area asynchronously.
 4. Thestructure production method according to claim 1, wherein in radiatingthe light onto the surface of the wafer, an irradiation cross-section ofthe light is moved over the surface of the wafer.
 5. A structureproduction method comprising preparing a wafer having a diameter of twoinches or more, at least a surface of the wafer being formed from agroup III nitride crystal, preparing an alkaline or acidic etchingliquid in a container, the etching liquid containing a peroxodisulfateion as an oxidizing agent that accepts an electron, accommodating thewafer in the container in a condition in which the surface of the waferis immersed in the etching liquid in a manner such that the surface ofthe wafer is parallel with a surface of the etching liquid, andradiating light onto the surface of the wafer from the surface side ofthe etching liquid heated so that a sulfate ion radical is generated,wherein the surface of the wafer includes a first etching area and asecond etching area, the first and second etching areas being disposedat an interval from each other, the first and second etching areas beingareas where the group III nitride crystal is to be etched due to thesurface of the wafer being irradiated with the light in a condition inwhich the surface is immersed in the etching liquid, and no mask isformed on the surface of the wafer, and in radiating the light onto thesurface of the wafer, the light is radiated perpendicularly onto each ofa surface of the first etching area and a surface of the second etchingarea.
 6. The structure production method according to claim 5, whereinin radiating the light onto the surface of the wafer, the light isradiated onto the first etching area and the second etching areasimultaneously.
 7. The structure production method according to claim 5,wherein in radiating the light onto the surface of the wafer, the lightis radiated onto the first etching area and the second etching areaasynchronously.
 8. The structure production method according to claim 5,wherein in radiating the light onto the surface of the wafer, anirradiation cross-section of the light is moved over the surface of thewafer.
 9. A structure production apparatus comprising a containerconfigured to accommodate a wafer and an alkaline or acidic etchingliquid, the wafer having a diameter of two inches or more, at least asurface of the wafer being formed from a group III nitride crystal, theetching liquid containing a peroxodisulfate ion as an oxidizing agentthat accepts an electron, a temperature adjuster configured to becapable of heating the etching liquid, and a light irradiation deviceconfigured to radiate light from a surface side of the etching liquidonto the surface of the wafer, wherein the container is configured toaccommodate the wafer inside the container in a condition in which thesurface of the wafer is immersed in the etching liquid in a manner suchthat the surface of the wafer is parallel with the surface of theetching liquid, and retain the etching liquid heated so that a sulfateion radical is generated in a manner such that the light is radiatedonto the surface of the wafer, and the light irradiation device isconfigured to radiate the light perpendicularly and selectively ontoetching areas of the surface of the wafer, the etching areas comprisinga first etching area and a second etching area, the first and secondetching areas being disposed at an interval from each other, the firstand second etching areas being areas where the group III nitride crystalis to be etched due to the surface of the wafer being irradiated withthe light in a condition in which the surface is immersed in the etchingliquid.
 10. The structure production apparatus according to claim 9,wherein the light irradiation device is configured to radiate the lightonto the first etching area and the second etching area simultaneously.11. The structure production apparatus according to claim 9, wherein thelight irradiation device is configured to radiate the light onto thefirst etching area and the second etching area asynchronously.
 12. Thestructure production apparatus according to claim 9, wherein the lightirradiation device is configured to move an irradiation cross-section ofthe light over the surface of the wafer.